By TM.

2009-04-17 20:09:26 8 Comments

ECMAScript 6 introduced the let statement.

I've heard it that it's described as a "local" variable, but I'm still not quite sure how it behaves differently than the var keyword.

What are the differences? When should let be used over var?


@Hasan Sefa Ozalp 2020-05-11 17:04:48

In most basic terms,

for (let i = 0; i < 5; i++) {
  // i accessible ✔️
// i not accessible ❌

for (var i = 0; i < 5; i++) {
  // i accessible ✔️
// i accessible ✔️

⚡️ Sandbox to play around ↓

Edit let vs var

@ThinkingStiff 2012-07-12 02:53:34

Scoping rules

Main difference is scoping rules. Variables declared by var keyword are scoped to the immediate function body (hence the function scope) while let variables are scoped to the immediate enclosing block denoted by { } (hence the block scope).

function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar);

    let baz = "Bazz";

  console.log(baz); // ReferenceError


The reason why let keyword was introduced to the language was function scope is confusing and was one of the main sources of bugs in JavaScript.

Take a look at this example from another stackoverflow question:

var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
  // and store them in funcs
  funcs[i] = function() {
    // each should log its value.
    console.log("My value: " + i);
for (var j = 0; j < 3; j++) {
  // and now let's run each one to see

My value: 3 was output to console each time funcs[j](); was invoked since anonymous functions were bound to the same variable.

People had to create immediately invoked functions to capture correct value from the loops but that was also hairy.


While variables declared with var keyword are hoisted (initialized with undefined before the code is run) which means they are accessible in their enclosing scope even before they are declared:

function run() {
  console.log(foo); // undefined
  var foo = "Foo";
  console.log(foo); // Foo


let variables are not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError. Variable said to be in "temporal dead zone" from the start of the block until the initialization is processed.

function checkHoisting() {
  console.log(foo); // ReferenceError
  let foo = "Foo";
  console.log(foo); // Foo


Creating global object property

At the top level, let, unlike var, does not create a property on the global object:

var foo = "Foo";  // globally scoped
let bar = "Bar"; // globally scoped

console.log(; // Foo
console.log(; // undefined


In strict mode, var will let you re-declare the same variable in the same scope while let raises a SyntaxError.

'use strict';
var foo = "foo1";
var foo = "foo2"; // No problem, 'foo' is replaced.

let bar = "bar1";
let bar = "bar2"; // SyntaxError: Identifier 'bar' has already been declared

@average Joe 2012-12-14 10:14:27

Remember you can create block whenever you want. function() { code;{ let inBlock = 5; } code; };

@NoBugs 2013-06-07 05:18:58

So is the purpose of let statements only to free up memory when not needed in a certain block?

@batman 2013-06-07 15:02:29

@NoBugs, Yes, and it is encouraged that variables are existent only where they are needed.

@Anvesh Checka 2014-09-26 03:19:02

let is not hoisted like its counterpart 'var' Hoisting behaves differently around a function block and any other block

@Gajus 2014-12-17 14:51:12

let block expression let (variable declaration) statement is non-standard and will be removed in future,

@Eric Bishard 2015-05-07 14:12:30

Although a let enclosing block is deprecated you can do the same thing by creating an explicit block with braces. { let bar = foo; let foo = bar; code; more code; console.log(bar + foo); } and simply have your lets at the top of your block of code wrapped in braces.

@Eric Bishard 2015-05-07 14:16:47

Just found this repo that allows you to use let blocks in your code. Because let blocks are pretty nice IMHO. which Transpiles non-ES6 let-blocks into ES6 (or ES3)

@zakius 2015-09-16 09:00:22

there are languages where you can use lone braces to create block (and probably scope too), does it work or is planned for js too? especially that let(variableName = "value") {block} is deprecated

@Toothbrush 2015-10-22 22:32:20

Note: As @EricB said, you can use let a = 1; { let a = 2; console.log(a === 2) } console.log(a === 1) in the place of let a = 1; let (a = 2) { console.log(a === 2) } console.log(a === 1). It does exactly the same.

@Luis Sieira 2015-11-08 13:12:08

So, I just cannot think of any case where using var is of any use. Could someone give me an example of a situation where it's preferable to use var?

@Dave Newton 2016-03-31 21:28:39

let at the top level scope is not identical to var -- let explicitly will not create globally-scoped references:…

@Chajnik-U 2017-05-08 07:29:00

Unfortunately browsers still (as of May 2017) do not optimize all corner cases for this nice feature and there are possible some loop slowdowns due to that…

@mormegil 2017-05-22 01:27:09

This answer does not address part of the question: "When should let be used over var?". I've attempted to answer that along with collecting a lot of the useful material in other answers below. If some kind soul is willing to see this put into the community wiki, I will not object to copy-paste in whole or in part.

@RBT 2017-06-01 01:08:00

I'm observing that redeclaration behavior remains same even without 'use strict'; mode. So it seems redeclaring a variable with same name using let keyword will always result in error no matter whether we are using strict mode or not. I'm using chrome version 58.

@Abdullah Khan 2018-03-22 08:24:20

This statement said it all The difference is scoping. var is scoped to the nearest function block and let is scoped to the nearest enclosing block Thanks

@Tim Hong 2018-04-23 02:29:09

I would like to correct that let me = 'go' is not truly under global scope like var me = 'go' if you try with let, you will get undefined. If you try with var, you will get me's value, in this case 'go'.

@Yuval A. 2019-01-05 19:17:22

Isn't recreating a new variable with each iteration of the loop, more expensive than using the same one for all iterations (like with var)?

@lrxw 2019-05-23 07:03:34

The result, that an undefined variable throws an exception if I want to use it before declaration the declaration with let (but not with var), surprises me. Doe this mean, that the JS Interpreter is looking ahead to know which variables are coming?

@qwr 2019-08-25 03:58:23

the "clever" variable names are distracting.

@SnnSnn 2019-09-11 11:36:03

I think "immediate" would be better term than nearest in "nearest function block".

@Aaron Franke 2019-12-06 08:59:09

Is there any disadvantage to just using let everywhere except if you specifically want var's behavior?

@Pete 2019-12-13 13:29:53

In the section about hoisting, it would make sense to add an example with a variable in global scope with the same name. In the first example, most people new to JS would be surprised that the global var is not logged (sorry if this has already been mentioned, there were too many comments to read them all)

@Srikrushna 2020-01-26 11:39:21

ES6 introduced two new keyword(let and const) alternate to var.

When you need a block level deceleration you can go with let and const instead of var.

The below table summarize the difference between var, let and const

enter image description here

@John Slegers 2016-02-23 18:35:27

What's the difference between let and var?

  • A variable defined using a var statement is known throughout the function it is defined in, from the start of the function. (*)
  • A variable defined using a let statement is only known in the block it is defined in, from the moment it is defined onward. (**)

To understand the difference, consider the following code:

// i IS NOT known here
// j IS NOT known here
// k IS known here, but undefined
// l IS NOT known here

function loop(arr) {
    // i IS known here, but undefined
    // j IS NOT known here
    // k IS known here, but has a value only the second time loop is called
    // l IS NOT known here

    for( var i = 0; i < arr.length; i++ ) {
        // i IS known here, and has a value
        // j IS NOT known here
        // k IS known here, but has a value only the second time loop is called
        // l IS NOT known here

    // i IS known here, and has a value
    // j IS NOT known here
    // k IS known here, but has a value only the second time loop is called
    // l IS NOT known here

    for( let j = 0; j < arr.length; j++ ) {
        // i IS known here, and has a value
        // j IS known here, and has a value
        // k IS known here, but has a value only the second time loop is called
        // l IS NOT known here

    // i IS known here, and has a value
    // j IS NOT known here
    // k IS known here, but has a value only the second time loop is called
    // l IS NOT known here


for( var k = 0; k < arr.length; k++ ) {
    // i IS NOT known here
    // j IS NOT known here
    // k IS known here, and has a value
    // l IS NOT known here

for( let l = 0; l < arr.length; l++ ) {
    // i IS NOT known here
    // j IS NOT known here
    // k IS known here, and has a value
    // l IS known here, and has a value


// i IS NOT known here
// j IS NOT known here
// k IS known here, and has a value
// l IS NOT known here

Here, we can see that our variable j is only known in the first for loop, but not before and after. Yet, our variable i is known in the entire function.

Also, consider that block scoped variables are not known before they are declared because they are not hoisted. You're also not allowed to redeclare the same block scoped variable within the same block. This makes block scoped variables less error prone than globally or functionally scoped variables, which are hoisted and which do not produce any errors in case of multiple declarations.

Is it safe to use let today?

Some people would argue that in the future we'll ONLY use let statements and that var statements will become obsolete. JavaScript guru Kyle Simpson wrote a very elaborate article on why he believes that won't be the case.

Today, however, that is definitely not the case. In fact, we need actually to ask ourselves whether it's safe to use the let statement. The answer to that question depends on your environment:

  • If you're writing server-side JavaScript code (Node.js), you can safely use the let statement.

  • If you're writing client-side JavaScript code and use a browser based transpiler (like Traceur or babel-standalone), you can safely use the let statement, however your code is likely to be anything but optimal with respect to performance.

  • If you're writing client-side JavaScript code and use a Node based transpiler (like the traceur shell script or Babel), you can safely use the let statement. And because your browser will only know about the transpiled code, performance drawbacks should be limited.

  • If you're writing client-side JavaScript code and don't use a transpiler, you need to consider browser support.

    There are still some browsers that don't support let at all :

enter image description here

How to keep track of browser support

For an up-to-date overview of which browsers support the let statement at the time of your reading this answer, see this Can I Use page.

(*) Globally and functionally scoped variables can be initialized and used before they are declared because JavaScript variables are hoisted. This means that declarations are always much to the top of the scope.

(**) Block scoped variables are not hoisted

@GitaarLAB 2016-05-21 04:41:11

regarding answer v4: i IS known everywhere in the function-block! It starts as undefined (due to hoisting) until you assign a value! ps: let is also hoisted (to the top of it's containing block), but will give a ReferenceError when referenced in the block before first assignment. (ps2: I'm a pro-semicolon kinda guy but you really don't need a semicolon after a block ). That being said, thanks for adding the reality-check regarding support!

@John Slegers 2018-02-26 23:37:27

@GitaarLAB : According to the Mozilla Developer Network : "In ECMAScript 2015, let bindings are not subject to Variable Hoisting, which means that let declarations do not move to the top of the current execution context." - Anyway, I made a few improvements to my answer that should clarify the difference in hoisting behavior between let and var!

@GitaarLAB 2018-03-01 18:16:42

Your answer improved a lot (I thoroughly checked). Note that same link you referenced in your comment also says: "The (let) variable is in a "temporal dead zone" from the start of the block until the initialization is processed." That means that the 'identifier' (the text-string 'reserved' to point to 'something') is already reserved in the relevant scope, otherwise it would become part of the root/host/window scope. To me personally, 'hoisting' means nothing more than reserving/linking declared 'identifiers' to their relevant scope; excluding their initialization/assignment/modifyability!

@GitaarLAB 2018-03-01 18:17:20

And..+1. That Kyle Simpson article you linked is an excellent read, thank you for that! It is also clear about the "temporal dead zone" aka "TDZ". One interesting thing I'd like to add: I've read on MDN that let and const were recommended to only use when you actually need their additional functionality, because enforcing/checking these extra features (like write-only const) result in 'more work' (and additional scope-nodes in the scope-tree) for the (current)engine(s) to enforce/check/verify/setup.

@Katinka Hesselink 2019-02-06 12:42:50

Note that MDN says that IE DOES interpret let correctly. Which is it?…

@John Slegers 2019-04-05 09:49:06

@KatinkaHesselink : I don't see the MDN saying any like that. As far as I can tell, they merely left out a quirk in IE11 that others have correctly documented. See also….

@jumps4fun 2019-11-07 10:47:57

Thank you for this answer. Everything is explained great in the two first sentences, and the addition of the table of supported browsers, really makes this a home run answer!

@Gurpreet Singh 2015-05-27 10:16:23

let can also be used to avoid problems with closures. It binds fresh value rather than keeping an old reference as shown in examples below.

for(var i=1; i<6; i++) {
  $("#div" + i).click(function () { console.log(i); });
<script src=""></script>
<p>Clicking on each number will log to console:</p> 
<div id="div1">1</div>
<div id="div2">2</div>
<div id="div3">3</div>
<div id="div4">4</div>
<div id="div5">5</div>

Code above demonstrates a classic JavaScript closure problem. Reference to the i variable is being stored in the click handler closure, rather than the actual value of i.

Every single click handler will refer to the same object because there’s only one counter object which holds 6 so you get six on each click.

A general workaround is to wrap this in an anonymous function and pass i as an argument. Such issues can also be avoided now by using let instead var as shown in the code below.

(Tested in Chrome and Firefox 50)

for(let i=1; i<6; i++) {
  $("#div" + i).click(function () { console.log(i); });
<script src=""></script>
<p>Clicking on each number will log to console:</p> 
<div id="div1">1</div>
<div id="div2">2</div>
<div id="div3">3</div>
<div id="div4">4</div>
<div id="div5">5</div>

@Karol Kolenda 2015-07-27 12:49:33

That is actually cool. I would expect "i" to be defined outside the loop body contains within brackets and to NOT form a "closure" around "i".Of course your example proves otherwise. I think it is a bit confusing from the syntax point of view but this scenario is so common it makes sense to support it in that way. Many thanks for bringing this up.

@Jim Hunziker 2015-10-22 13:29:07

IE 11 supports let, but it alerts "6" for all the buttons. Do you have any source saying how let is supposed to behave?

@Jim Hunziker 2015-10-22 13:32:39

Looks like your answer is the correct behavior:…

@Marie 2016-01-28 14:25:28

While reading the link in @Jim s comment I noticed a note that mentioned let working in Firefox 44.

@Adrian Moisa 2016-02-21 08:12:08

Indeed this is a common pitfall in Javascript and now I can see why let would be really useful. Setting event listeners in a loop no longer requires an immediatelly invoked function expression for locally scoping i at each iteration.

@gary 2016-09-07 23:10:20

The use of "let" just defers this problem. So each iteration creates a private independent block scope, but the "i" variable can still be corrupted by subsequent changes within the block, (granted the iterator variable is not usually changed within the block, but other declared let variables within the block may well be) and any function declared within the block can, when invoked, corrupt the value of "i" for other functions declared within the block because they do share the same private block scope hence the same reference to "i".

@krulik 2017-03-20 11:57:52

@gary According to MDN + changing the above demo to i++ each function has its own block scope and doesn't interfere with the other instances:

@gary 2017-03-31 16:32:09

@krulik This is what I meant: two functions declared in the same block modify the shared loop variable i.

@giorgio-b 2018-03-08 13:52:42

for(let i = 1; i < 6; i++) I personally find this notation misleading: i++ suggests that i is increased by 1. Instead, a new variable named i is passed to the next iteration and initialized the the value of i in the previous iteration increased by 1. I think they should have chosen another notation.

@jrh 2018-12-26 15:14:42

For readers of "Javascript: The Good Parts", note that using let instead of var in the for loop (as in the second code block in this answer) prevents the code from showing an alert containing only the number of nodes; here's a link to the example code.

@Ankit Deshpande 2019-01-25 09:20:35

@giorgio-b does this mean each iteration of the loop will create a new variable with increment value of i ?

@Andrew 2019-09-09 20:25:05

Uhh... I'm getting 12345 for both examples... FF 69.0

@ToolmakerSteve 2019-10-15 12:11:06

@gary - that's exactly what should happen. i is available to all nested code. Without this, there is no easy way to refer to variables in outer scopes. If those two functions want independent is, then they each declare their own, using let. OTOH I see your point - there is no way to ensure i is "frozen" (constant) within the loop. Could assign it to a const: const j = i;, but still have to manually verify that i isn't referenced anywhere.

@Kirby L. Wallace 2019-10-23 17:56:02

Has something changed in Javascript since this answer was posted? Both the "var" and "let" version produce the same results.

@Piklu Dey 2019-09-07 11:25:15

The below shows how 'let' and 'var' are different in the scope:

let gfoo = 123;
if (true) {
    let gfoo = 456;
console.log(gfoo); // 123

var hfoo = 123;
if (true) {
    var hfoo = 456;
console.log(hfoo); // 456

The gfoo, defined by let initially is in the global scope, and when we declare gfoo again inside the if clause its scope changed and when a new value is assigned to the variable inside that scope it does not affect the global scope.

Whereas hfoo, defined by var is initially in the global scope, but again when we declare it inside the if clause, it considers the global scope hfoo, although var has been used again to declare it. And when we re-assign its value we see that the global scope hfoo is also affected. This is the primary difference.

@olliej 2009-04-17 21:38:22

There are some subtle differences — let scoping behaves more like variable scoping does in more or less any other languages.

e.g. It scopes to the enclosing block, They don't exist before they're declared, etc.

However it's worth noting that let is only a part of newer Javascript implementations and has varying degrees of browser support.

@Richard Ayotte 2012-03-31 15:09:58

It's also worth noting that ECMAScript is the standard and let is included in the 6th edition draft and will most likely be in the final specification.

@olliej 2012-04-13 03:28:43

That's the difference 3 years makes :D

@pseudosavant 2012-07-13 17:38:18

Just stubled across this question and in 2012 it is still the case that only Mozilla browsers support let. Safari, IE, and Chome all don't.

@Eric Bishard 2015-05-07 14:01:33

The idea of accidentally creating partial block scope on accident is a good point, beware, let does not hoist, to use a variable defined by a let defined at the top of your block. If you have an if statement that is more than just a few lines of code, you may forget that you cannot use that variable until after it is defined. GREAT POINT!!!

@Jay 2015-06-21 16:12:13

This is one of the most important distinctions between let and var and it's not in the accepted answer haha. Especially considering the numerous bugs that can occur thanks to hoisting and scoping. I feel like there aren't many differences between let and var if you don't mention hoisting.

@GitaarLAB 2016-05-21 04:15:53

@EricB: yes and no: "In ECMAScript 2015, let will hoist the variable to the top of the block. However, referencing the variable in the block before the variable declaration results in a ReferenceError (my note: instead of good old undefined). The variable is in a 'temporal dead zone' from the start of the block until the declaration is processed." Same goes for "switch statements because there is only one underlying block". Source:…

@Ben S 2009-04-17 20:11:47

Here's an explanation of the let keyword with some examples.

let works very much like var. The main difference is that the scope of a var variable is the entire enclosing function

This table on Wikipedia shows which browsers support Javascript 1.7.

Note that only Mozilla and Chrome browsers support it. IE, Safari, and potentially others don't.

@Michael Burr 2009-04-17 20:25:25

The key bit of text from the linked document seems to be, "let works very much like var. The main difference is that the scope of a var variable is the entire enclosing function".

@olliej 2009-04-17 22:56:39

While it's technically correct to say IE does not support it, it's more correct to say that it's a mozilla only extension.

@Tyler Crompton 2012-06-18 20:16:29

@olliej, actually Mozilla is just ahead of the game. See page 19 of‌​df

@olliej 2012-07-11 18:49:45

@TylerCrompton that's just the set of words that have been reserved for years. When mozilla added let it was purely a mozilla extension, with no related spec. ES6 should define behaviour for let statements, but that came after mozilla introduced the syntax. Remember moz also has E4X, which is entirely dead and moz only.

@eloyesp 2013-12-24 12:59:15

@Shapon Pal 2019-01-08 05:11:33

Now let support all latest browser today except Opera, Blackberry & QQ Browsers.

@vlio20 2015-03-06 10:41:10

Here is an example for the difference between the two (support just started for chrome):
enter image description here

As you can see the var j variable is still having a value outside of the for loop scope (Block Scope), but the let i variable is undefined outside of the for loop scope.

"use strict";
for (var j = 0; j < 2; j++) {


for (let i = 0; i < 2; i++) {


@Barton 2015-03-24 21:43:48

What tool am I looking at here?

@vlio20 2015-03-24 21:44:52

Chrome devtools

@Barton 2017-10-26 04:20:30

As a developer of desktop applets for Cinnamon, I haven't been exposed to such shiny tools.

@Lucian Nut 2019-03-11 16:52:45

As I am currently trying to get an in depth understanding of JavaScript I will share my brief research which contains some of the great pieces already discussed plus some other details in a different perspective.

Understanding the difference between var and let can be easier if we understand the difference between function and block scope.

Let's consider the following cases:

(function timer() {
    for(var i = 0; i <= 5; i++) {
        setTimeout(function notime() { console.log(i); }, i * 1000);

   Stack            VariableEnvironment //one VariablEnvironment for timer();
                                       // when the timer is out - the value will be the same value for each call
5. [setTimeout, i]  [i=5] 
4. [setTimeout, i]  
3. [setTimeout, i]
2. [setTimeout, i]
1. [setTimeout, i]
0. [setTimeout, i]


(function timer() {
    for (let i = 0; i <= 5; i++) {
        setTimeout(function notime() { console.log(i); }, i * 1000);

   Stack           LexicalEnvironment - each iteration has a new lexical environment
5. [setTimeout, i]  [i=5]       
4. [setTimeout, i]    [i=4]     
3. [setTimeout, i]      [i=3]       
2. [setTimeout, i]       [i=2]
1. [setTimeout, i]         [i=1]
0. [setTimeout, i]           [i=0]

when timer() gets called an ExecutionContext is created which will contain both the VariableEnvironment and all the LexicalEnvironments corresponding to each iteration.

And a simpler example

Function Scope

function test() {
    for(var z = 0; z < 69; z++) {
    //z is visible outside the loop

Block Scope

function test() {
    for(let z = 0; z < 69; z++) {
    //z is not defined :(

@Dementic 2019-04-28 02:21:13

I think the terms and most of the examples are a bit overwhelming, The main issue i had personally with the difference is understanding what a "Block" is. At some point i realized, a block would be any curly brackets except for IF statement. an opening bracket { of a function or loop will define a new block, anything defined with let within it, will not be available after the closing bracket } of the same thing (function or loop); With that in mind, it was easier to understand:

let msg = "Hello World";

function doWork() { // msg will be available since it was defined above this opening bracket!
  let friends = 0;

  // with VAR though:
  for (var iCount2 = 0; iCount2 < 5; iCount2++) {} // iCount2 will be available after this closing bracket!
    for (let iCount1 = 0; iCount1 < 5; iCount1++) {} // iCount1 will not be available behind this closing bracket, it will return undefined
} // friends will no be available after this closing bracket!

@daCoda 2019-04-18 00:49:32

let vs var. It's all about scope.

var variables are global and can be accessed basically everywhere, while let variables are not global and only exist until a closing parenthesis kills them.

See my example below, and note how the lion (let) variable acts differently in the two console.logs; it becomes out of scope in the 2nd console.log.

var cat = "cat";
let dog = "dog";

var animals = () => {
    var giraffe = "giraffe";
    let lion = "lion";

    console.log(cat);  //will print 'cat'.
    console.log(dog);  //will print 'dog', because dog was declared outside this function (like var cat).

    console.log(giraffe); //will print 'giraffe'.
    console.log(lion); //will print 'lion', as lion is within scope.

console.log(giraffe); //will print 'giraffe', as giraffe is a global variable (var).
console.log(lion); //will print UNDEFINED, as lion is a 'let' variable and is now out of scope.

@Mile Mijatović 2019-02-16 17:17:45

enter image description here

Take a look at this image, I created one very simple example for demonstration of const and let variables. As you can see, when you try to change const variable, you will get the error (Attempting to override 'name' which is constant'), but take a look at let variable...

First we declare let age = 33, and later assign some other value age = 34;, which is ok, we dont have any errors when we try to change let variable

@Donato 2019-02-13 16:07:22

I want to link these keywords to the Execution Context, because the Execution Context is important in all of this. The Execution Context has two phases: a Creation Phase and Execution Phase. In addition, each Execution Context has a Variable Environment and Outer Environment (its Lexical Environment).

During the Creation Phase of an Execution Context, var, let and const will still store its variable in memory with an undefined value in the Variable Environment of the given Execution Context. The difference is in the Execution Phase. If you use reference a variable defined with var before it is assigned a value, it will just be undefined. No exception will be raised.

However, you cannot reference the variable declared with let or const until it is declared. If you try to use it before it is declared, then an exception will be raised during the Execution Phase of the Execution Context. Now the variable will still be in memory, courtesy of the Creation Phase of the Execution Context, but the Engine will not allow you to use it:

function a(){
    let b;
> Uncaught ReferenceError: b is not defined

With a variable defined with var, if the Engine cannot find the variable in the current Execution Context's Variable Environment, then it will go up the scope chain (the Outer Environment) and check the Outer Environment's Variable Environment for the variable. If it cannot find it there, it will continue searching the Scope Chain. This is not the case with let and const.

The second feature of let is it introduces block scope. Blocks are defined by curly braces. Examples include function blocks, if blocks, for blocks, etc. When you declare a variable with let inside of a block, the variable is only available inside of the block. In fact, each time the block is run, such as within a for loop, it will create a new variable in memory.

ES6 also introduces the const keyword for declaring variables. const is also block scoped. The difference between let and const is that const variables need to be declared using an initializer, or it will generate an error.

And, finally, when it comes to the Execution Context, variables defined with var will be attached to the 'this' object. In the global Execution Context, that will be the window object in browsers. This is not the case for let or const.

@Alireza 2017-03-22 14:39:46

The main difference is the scope difference, while let can be only available inside the scope it's declared, like in for loop, var can be accessed outside the loop for example. From the documentation in MDN (examples also from MDN):

let allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used. This is unlike the var keyword, which defines a variable globally, or locally to an entire function regardless of block scope.

Variables declared by let have as their scope the block in which they are defined, as well as in any contained sub-blocks. In this way, let works very much like var. The main difference is that the scope of a var variable is the entire enclosing function:

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // same variable!
    console.log(x);  // 2
  console.log(x);  // 2

function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // different variable
    console.log(x);  // 2
  console.log(x);  // 1

At the top level of programs and functions, let, unlike var, does not create a property on the global object. For example:

var x = 'global';
let y = 'global';
console.log(this.x); // "global"
console.log(this.y); // undefined

When used inside a block, let limits the variable's scope to that block. Note the difference between var whose scope is inside the function where it is declared.

var a = 1;
var b = 2;

if (a === 1) {
  var a = 11; // the scope is global
  let b = 22; // the scope is inside the if-block

  console.log(a);  // 11
  console.log(b);  // 22

console.log(a); // 11
console.log(b); // 2

Also don't forget it's ECMA6 feature, so it's not fully supported yet, so it's better always transpiles it to ECMA5 using Babel etc... for more info about visit babel website

@Daniel Sokolowski 2016-10-14 05:01:10

If I read the specs right then let thankfully can also be leveraged to avoid self invoking functions used to simulate private only members - a popular design pattern that decreases code readability, complicates debugging, that adds no real code protection or other benefit - except maybe satisfying someone's desire for semantics, so stop using it. /rant

var SomeConstructor;

    let privateScope = {};

    SomeConstructor = function SomeConstructor () {
        this.someProperty = "foo";
        privateScope.hiddenProperty = "bar";

    SomeConstructor.prototype.showPublic = function () {
        console.log(this.someProperty); // foo

    SomeConstructor.prototype.showPrivate = function () {
        console.log(privateScope.hiddenProperty); // bar


var myInstance = new SomeConstructor();


console.log(privateScope.hiddenProperty); // error

See 'Emulating private interfaces'

@Robert Siemer 2020-03-01 03:58:54

Can you elaborate on how Immediately Invoked Function Expressions do not provide “code protection” and let does? (I assume you mean IIFE with “self invoking function”.)

@Robert Siemer 2020-03-01 12:52:29

And why do you set hiddenProperty in the constructor? There is only one hiddenProperty for all instances in your “class”.

@Michał Perłakowski 2016-11-23 22:52:38


Block scope

Variables declared using the let keyword are block-scoped, which means that they are available only in the block in which they were declared.

At the top level (outside of a function)

At the top level, variables declared using let don't create properties on the global object.

var globalVariable = 42;
let blockScopedVariable = 43;

console.log(globalVariable); // 42
console.log(blockScopedVariable); // 43

console.log(this.globalVariable); // 42
console.log(this.blockScopedVariable); // undefined

Inside a function

Inside a function (but outside of a block), let has the same scope as var.

(() => {
  var functionScopedVariable = 42;
  let blockScopedVariable = 43;

  console.log(functionScopedVariable); // 42
  console.log(blockScopedVariable); // 43

console.log(functionScopedVariable); // ReferenceError: functionScopedVariable is not defined
console.log(blockScopedVariable); // ReferenceError: blockScopedVariable is not defined

Inside a block

Variables declared using let inside a block can't be accessed outside that block.

  var globalVariable = 42;
  let blockScopedVariable = 43;
  console.log(globalVariable); // 42
  console.log(blockScopedVariable); // 43

console.log(globalVariable); // 42
console.log(blockScopedVariable); // ReferenceError: blockScopedVariable is not defined

Inside a loop

Variables declared with let in loops can be referenced only inside that loop.

for (var i = 0; i < 3; i++) {
  var j = i * 2;
console.log(i); // 3
console.log(j); // 4

for (let k = 0; k < 3; k++) {
  let l = k * 2;
console.log(typeof k); // undefined
console.log(typeof l); // undefined
// Trying to do console.log(k) or console.log(l) here would throw a ReferenceError.

Loops with closures

If you use let instead of var in a loop, with each iteration you get a new variable. That means that you can safely use a closure inside a loop.

// Logs 3 thrice, not what we meant.
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);

// Logs 0, 1 and 2, as expected.
for (let j = 0; j < 3; j++) {
  setTimeout(() => console.log(j), 0);

Temporal dead zone

Because of the temporal dead zone, variables declared using let can't be accessed before they are declared. Attempting to do so throws an error.

console.log(noTDZ); // undefined
var noTDZ = 43;
console.log(hasTDZ); // ReferenceError: hasTDZ is not defined
let hasTDZ = 42;

No re-declaring

You can't declare the same variable multiple times using let. You also can't declare a variable using let with the same identifier as another variable which was declared using var.

var a;
var a; // Works fine.

let b;
let b; // SyntaxError: Identifier 'b' has already been declared

var c;
let c; // SyntaxError: Identifier 'c' has already been declared


const is quite similar to let—it's block-scoped and has TDZ. There are, however, two things which are different.

No re-assigning

Variable declared using const can't be re-assigned.

const a = 42;
a = 43; // TypeError: Assignment to constant variable.

Note that it doesn't mean that the value is immutable. Its properties still can be changed.

const obj = {};
obj.a = 42;
console.log(obj.a); // 42

If you want to have an immutable object, you should use Object.freeze().

Initializer is required

You always must specify a value when declaring a variable using const.

const a; // SyntaxError: Missing initializer in const declaration

@Willem van der Veen 2018-09-09 13:08:18

Function VS block scope:

The main difference between var and let is that variables declared with var are function scoped. Whereas functions declared with let are block scoped. For example:

function testVar () {
  if(true) {
    var foo = 'foo';


// logs 'foo'

function testLet () {
  if(true) {
    let bar = 'bar';


// reference error
// bar is scoped to the block of the if statement 

variables with var:

When the first function testVar gets called the variable foo, declared with var, is still accessible outside the if statement. This variable foo would be available everywhere within the scope of the testVar function.

variables with let:

When the second function testLet gets called the variable bar, declared with let, is only accessible inside the if statement. Because variables declared with let are block scoped (where a block is the code between curly brackets e.g if{} , for{}, function{}).

let variables don't get hoisted:

Another difference between var and let is variables with declared with let don't get hoisted. An example is the best way to illustrate this behavior:

variables with let don't get hoisted:


let letVar = 10;
// referenceError, the variable doesn't get hoisted

variables with var do get hoisted:


var varVar = 10;
// logs undefined, the variable gets hoisted

Global let doesn't get attached to window:

A variable declared with let in the global scope (which is code that is not in a function) doesn't get added as a property on the global window object. For example (this code is in global scope):

var bar = 5;
let foo  = 10;

console.log(bar); // logs 5
console.log(foo); // logs 10

// logs 5, variable added to window object

// logs undefined, variable not added to window object

When should let be used over var?

Use let over var whenever you can because it is simply scoped more specific. This reduces potential naming conflicts which can occur when dealing with a large number of variables. var can be used when you want a global variable explicitly to be on the window object (always consider carefully if this is really necessary).

@N Randhawa 2018-08-07 10:25:57

As mentioned above:

The difference is scoping. var is scoped to the nearest function block and let is scoped to the nearest enclosing block, which can be smaller than a function block. Both are global if outside any block.Lets see an example:


In my both examples I have a function myfunc. myfunc contains a variable myvar equals to 10. In my first example I check if myvar equals to 10 (myvar==10) . If yes, I agian declare a variable myvar (now I have two myvar variables)using var keyword and assign it a new value (20). In next line I print its value on my console. After the conditional block I again print the value of myvar on my console. If you look at the output of myfunc, myvar has value equals to 20.

let keyword

Example2: In my second example instead of using var keyword in my conditional block I declare myvar using let keyword . Now when I call myfunc I get two different outputs: myvar=20 and myvar=10.

So the difference is very simple i.e its scope.

@inostia 2018-08-24 17:29:59

Please don't post pictures of code, it's considered bad practice on SO as it will not be searchable for future users (as well as accessibility concerns). As well, this answer adds nothing that other answers haven't already addressed.

@Ankur Soni 2018-05-22 13:12:36

When Using let

The let keyword attaches the variable declaration to the scope of whatever block (commonly a { .. } pair) it's contained in. In other words,let implicitly hijacks any block's scope for its variable declaration.

let variables cannot be accessed in the window object because they cannot be globally accessed.

function a(){
    { // this is the Max Scope for let variable
        let x = 12;
a(); // Uncaught ReferenceError: x is not defined

When Using var

var and variables in ES5 has scopes in functions meaning the variables are valid within the function and not outside the function itself.

var variables can be accessed in the window object because they cannot be globally accessed.

function a(){ // this is the Max Scope for var variable
        var x = 12;
a(); // 12

If you want to know more continue reading below

one of the most famous interview questions on scope also can suffice the exact use of let and var as below;

When using let

for (let i = 0; i < 10 ; i++) {
        function a() {
            console.log(i); //print 0 to 9, that is literally AWW!!!
        100 * i);

This is because when using let, for every loop iteration the variable is scoped and has its own copy.

When using var

for (var i = 0; i < 10 ; i++) {
        function a() {
            console.log(i); //print 10 times 10
        100 * i);

This is because when using var, for every loop iteration the variable is scoped and has shared copy.

@Nurlan 2018-05-18 07:27:41

Check this link in MDN

let x = 1;

if (x === 1) {
let x = 2;

// expected output: 2

// expected output: 1

@pushkin 2018-05-31 19:47:10

It would additionally help to explain in words what let is doing, though that would make it a duplicate of another answer here

@Thomas Darvik 2018-09-27 09:41:21

What he is trying to say is that the let outside the if defines the variable x=1. The if-statement will trigger (since x===1 is true. Now comes the tricky part, which is also the main reason you need to keep track of let vs var. Inside the if he is trying to set x=2, which in the console.log(x) will print 2, HOWEVER, x "on the outside" of the if still has has the value 1, so the other console.log(x) gives 1, since the value of x "globally" is still 1. The answer does not explain this, so should not be considered a good answer in my opinion.

@vipul jain 2017-12-17 10:47:31

let is a part of es6. These functions will explain the difference in easy way.

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // same variable!
    console.log(x);  // 2
  console.log(x);  // 2

function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // different variable
    console.log(x);  // 2
  console.log(x);  // 1

@Moslem Shahsavan 2017-10-28 12:42:16

var is global scope (hoist-able) variable.

let and const is block scope.


    let l = 'let';
    const c = 'const';
    var v = 'var';
    v2 = 'var 2';

console.log(v, this.v);
console.log(v2, this.v2);
console.log(l); // ReferenceError: l is not defined
console.log(c); // ReferenceError: c is not defined

@abroz 2014-08-18 00:58:29

Here's an example to add on to what others have already written. Suppose you want to make an array of functions, adderFunctions, where each function takes a single Number argument and returns the sum of the argument and the function's index in the array. Trying to generate adderFunctions with a loop using the var keyword won't work the way someone might naïvely expect:

// An array of adder functions.
var adderFunctions = [];

for (var i = 0; i < 1000; i++) {
  // We want the function at index i to add the index to its argument.
  adderFunctions[i] = function(x) {
    // What is i bound to here?
    return x + i;

var add12 = adderFunctions[12];

// Uh oh. The function is bound to i in the outer scope, which is currently 1000.
console.log(add12(8) === 20); // => false
console.log(add12(8) === 1008); // => true
console.log(i); // => 1000

// It gets worse.
i = -8;
console.log(add12(8) === 0); // => true

The process above doesn't generate the desired array of functions because i's scope extends beyond the iteration of the for block in which each function was created. Instead, at the end of the loop, the i in each function's closure refers to i's value at the end of the loop (1000) for every anonymous function in adderFunctions. This isn't what we wanted at all: we now have an array of 1000 different functions in memory with exactly the same behavior. And if we subsequently update the value of i, the mutation will affect all the adderFunctions.

However, we can try again using the let keyword:

// Let's try this again.
// NOTE: We're using another ES6 keyword, const, for values that won't
// be reassigned. const and let have similar scoping behavior.
const adderFunctions = [];

for (let i = 0; i < 1000; i++) {
  // NOTE: We're using the newer arrow function syntax this time, but 
  // using the "function(x) { ..." syntax from the previous example 
  // here would not change the behavior shown.
  adderFunctions[i] = x => x + i;

const add12 = adderFunctions[12];

// Yay! The behavior is as expected. 
console.log(add12(8) === 20); // => true

// i's scope doesn't extend outside the for loop.
console.log(i); // => ReferenceError: i is not defined

This time, i is rebound on each iteration of the for loop. Each function now keeps the value of i at the time of the function's creation, and adderFunctions behaves as expected.

Now, image mixing the two behaviors and you'll probably see why it's not recommended to mix the newer let and const with the older var in the same script. Doing so can result is some spectacularly confusing code.

const doubleAdderFunctions = [];

for (var i = 0; i < 1000; i++) {
    const j = i;
    doubleAdderFunctions[i] = x => x + i + j;

const add18 = doubleAdderFunctions[9];
const add24 = doubleAdderFunctions[12];

// It's not fun debugging situations like this, especially when the
// code is more complex than in this example.
console.log(add18(24) === 42); // => false
console.log(add24(18) === 42); // => false
console.log(add18(24) === add24(18)); // => false
console.log(add18(24) === 2018); // => false
console.log(add24(18) === 2018); // => false
console.log(add18(24) === 1033); // => true
console.log(add24(18) === 1030); // => true

Don't let this happen to you. Use a linter.

NOTE: This is a teaching example intended to demonstrate the var/let behavior in loops and with function closures that would also be easy to understand. This would be a terrible way to add numbers. But the general technique of capturing data in anonymous function closures might be encountered in the real world in other contexts. YMMV.

@Barton 2015-02-20 08:59:24

@aborz: Also very cool anonymous function syntax in the second example. It's just what I'm used to in C#. I've learned something today.

@Barton 2015-03-16 06:58:17

Correction: Technically, Arrow function syntax described here =>…

@Toothbrush 2015-10-22 22:38:54

Actually, you don't need let value = i;. The for statement creates a lexical block.

@mormegil 2017-05-22 01:09:39

The difference is in the scope of the variables declared with each.

In practice, there are a number of useful consequences of the difference in scope:

  1. let variables are only visible in their nearest enclosing block ({ ... }).
  2. let variables are only usable in lines of code that occur after the variable is declared (even though they are hoisted!).
  3. let variables may not be redeclared by a subsequent var or let.
  4. Global let variables are not added to the global window object.
  5. let variables are easy to use with closures (they do not cause race conditions).

The restrictions imposed by let reduce the visibility of the variables and increase the likelihood that unexpected name collisions will be found early. This makes it easier to track and reason about variables, including their reachability(helping with reclaiming unused memory).

Consequently, let variables are less likely to cause problems when used in large programs or when independently-developed frameworks are combined in new and unexpected ways.

var may still be useful if you are sure you want the single-binding effect when using a closure in a loop (#5) or for declaring externally-visible global variables in your code (#4). Use of var for exports may be supplanted if export migrates out of transpiler space and into the core language.


1. No use outside nearest enclosing block: This block of code will throw a reference error because the second use of x occurs outside of the block where it is declared with let:

    let x = 1;
console.log(`x is ${x}`);  // ReferenceError during parsing: "x is not defined".

In contrast, the same example with var works.

2. No use before declaration:
This block of code will throw a ReferenceError before the code can be run because x is used before it is declared:

    x = x + 1;  // ReferenceError during parsing: "x is not defined".
    let x;
    console.log(`x is ${x}`);  // Never runs.

In contrast, the same example with var parses and runs without throwing any exceptions.

3. No redeclaration: The following code demonstrates that a variable declared with let may not be redeclared later:

let x = 1;
let x = 2;  // SyntaxError: Identifier 'x' has already been declared

4. Globals not attached to window:

var button = "I cause accidents because my name is too common.";
let link = "Though my name is common, I am harder to access from other JS files.";
console.log(link);  // OK
console.log(;  // undefined (GOOD!)
console.log(window.button);  // OK

5. Easy use with closures: Variables declared with var do not work well with closures inside loops. Here is a simple loop that outputs the sequence of values that the variable i has at different points in time:

for (let i = 0; i < 5; i++) {
    console.log(`i is ${i}`), 125/*ms*/);

Specifically, this outputs:

i is 0
i is 1
i is 2
i is 3
i is 4

In JavaScript we often use variables at a significantly later time than when they are created. When we demonstrate this by delaying the output with a closure passed to setTimeout:

for (let i = 0; i < 5; i++) {
    setTimeout(_ => console.log(`i is ${i}`), 125/*ms*/);

... the output remains unchanged as long as we stick with let. In contrast, if we had used var i instead:

for (var i = 0; i < 5; i++) {
    setTimeout(_ => console.log(`i is ${i}`), 125/*ms*/);

... the loop unexpectedly outputs "i is 5" five times:

i is 5
i is 5
i is 5
i is 5
i is 5

@Daniel T. 2017-06-02 01:12:01

#5 is not caused by a race condition. By using var instead of let, the code is equivalent to: var i = 0; while (i < 5) { doSomethingLater(); i++; } i is outside the closure, and by the time that doSomethingLater() is executed, i has already been incremented 5 times, hence the output is i is 5 five times. By using let, the variable i is within the closure, so each async call gets its own copy of i instead of using the 'global' one that's created with var.

@mormegil 2017-07-25 07:13:26

@DanielT.: I don't think the transformation of lifting the variable definition out of the loop initializer explains anything. That is simply the normal definition of the semantics of for. A more accurate transformation, though more complicated, is the classical for (var i = 0; i < 5; i++) { (function(j) { setTimeout(_ => console.log(i is ${j}), 125/*ms*/); })(i); } which introduces a "function-activation record" to save each value of i with the name of j inside the function.

@anandharshan 2016-12-27 09:44:55

This article clearly defines the difference between var, let and const

const is a signal that the identifier won’t be reassigned.

let, is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it’s defined in, which is not always the entire containing function.

var is now the weakest signal available when you define a variable in JavaScript. The variable may or may not be reassigned, and the variable may or may not be used for an entire function, or just for the purpose of a block or loop.

@zangw 2016-01-17 15:11:31

  • Variable Not Hoisting

    let will not hoist to the entire scope of the block they appear in. By contrast, var could hoist as below.

       console.log(cc); // undefined. Caused by hoisting
       var cc = 23;
       console.log(bb); // ReferenceError: bb is not defined
       let bb = 23;

    Actually, Per @Bergi, Both var and let are hoisted.

  • Garbage Collection

    Block scope of let is useful relates to closures and garbage collection to reclaim memory. Consider,

    function process(data) {
    var hugeData = { .. };
    var btn = document.getElementById("mybutton");
    btn.addEventListener( "click", function click(evt){

    The click handler callback does not need the hugeData variable at all. Theoretically, after process(..) runs, the huge data structure hugeData could be garbage collected. However, it's possible that some JS engine will still have to keep this huge structure, since the click function has a closure over the entire scope.

    However, the block scope can make this huge data structure to garbage collected.

    function process(data) {
    { // anything declared inside this block can be garbage collected
        let hugeData = { .. };
    var btn = document.getElementById("mybutton");
    btn.addEventListener( "click", function click(evt){
  • let loops

    let in the loop can re-binds it to each iteration of the loop, making sure to re-assign it the value from the end of the previous loop iteration. Consider,

    // print '5' 5 times
    for (var i = 0; i < 5; ++i) {
        setTimeout(function () {
        }, 1000);  

    However, replace var with let

    // print 1, 2, 3, 4, 5. now
    for (let i = 0; i < 5; ++i) {
        setTimeout(function () {
        }, 1000);  

    Because let create a new lexical environment with those names for a) the initialiser expression b) each iteration (previosly to evaluating the increment expression), more details are here.

@Drenai 2016-12-31 15:42:05

Yip they are hoisted, but behave as if not hoisted because of the (drum roll) Temporal Dead Zone - a very dramatic name for an identifier not being accessible until it's declared:-)

@N-ate 2017-11-21 21:46:30

So let is hoisted, but unavailable? How is that different than 'not hoisted'?

@N-ate 2017-11-22 16:18:57

Hopefully Brian or Bergi come back to answer this. Is the declaration of let hoisted, but not the assignment? Thanks!

@zangw 2017-11-23 03:46:46

@N-ate, Here is one post of Bergi, maybe you can find answer in it.

@N-ate 2017-11-23 23:49:43

It's interesting it is even called hoisting when it comes to let. I get that technically the parsing engine is pre-capturing it, but for all intents and purposes a programmer should treat it as if it doesn't exist. The hoisting of var on the other hand has implications to a programmer.

@RDoc 2015-08-11 00:35:29

It also appears that, at least in Visual Studio 2015, TypeScript 1.5, "var" allows multiple declarations of the same variable name in a block, and "let" doesn't.

This won't generate a compile error:

var x = 1;
var x = 2;

This will:

let x = 1;
let x = 2;

@zloctb 2016-07-21 17:42:16

Some hacks with let:


    let statistics = [16, 170, 10];
    let [age, height, grade] = statistics;



    let x = 120,
    y = 12;
    [x, y] = [y, x];
    console.log(`x: ${x} y: ${y}`);


    let node = {
                   type: "Identifier",
                   name: "foo"

    let { type, name, value } = node;

    console.log(type);      // "Identifier"
    console.log(name);      // "foo"
    console.log(value);     // undefined

    let node = {
        type: "Identifier"

    let { type: localType, name: localName = "bar" } = node;

    console.log(localType);     // "Identifier"
    console.log(localName);     // "bar"

Getter and setter with let:

let jar = {
    numberOfCookies: 10,
    get cookies() {
        return this.numberOfCookies;
    set cookies(value) {
        this.numberOfCookies = value;

jar.cookies = 7;


@AlainIb 2017-06-15 07:55:34

please what do this mean let { type, name, value } = node; ? you create a new object with 3 properties type/name/value and initialise them with the properties values from node ?

@Rehan Haider 2019-01-09 10:57:08

In example 3 you are re-declaring node which cause exception. These all examples also work perfectly with var too.

@Dmitry 2016-07-08 00:21:11

let is interesting, because it allows us to do something like this:

(() => {
    var count = 0;

    for (let i = 0; i < 2; ++i) {
        for (let i = 0; i < 2; ++i) {
            for (let i = 0; i < 2; ++i) {

Which results in counting [0, 7].


(() => {
    var count = 0;

    for (var i = 0; i < 2; ++i) {
        for (var i = 0; i < 2; ++i) {
            for (var i = 0; i < 2; ++i) {

Only counts [0, 1].

@John Haugeland 2016-11-24 00:37:34

this is the first time i've ever seen anyone act like variable shadowing was desirable. no, the purpose of let is not to enable shadowing

@Dmitry 2016-11-24 00:39:05

purpose? it's a construct, you can use it however you please, one of the interesting ways is like this.

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