By gramm

2009-11-24 13:04:29 8 Comments

Usually I would expect a String.contains() method, but there doesn't seem to be one.

What is a reasonable way to check for this?


@Fabien Ménager 2009-11-24 13:05:36


ES6 introduced String.prototype.includes:

'foo'.includes('oo') // true
'foo'.includes('bar') // false


includes doesn’t have IE support, though. In an ES5 or older environment, String.prototype.indexOf, which returns −1 when it doesn’t find the substring, can be used instead:

'foo'.indexOf('oo') // 1
'foo'.indexOf('bar') // -1

@Mark Amery 2019-04-16 13:58:58

@dimpiax, you've restored the example using match that Ry- deliberately chose to remove. Why? It's not necessarily wrong to have it here, but what advantage does it have over indexOf? What's the motivation for ever using it over one of the other two options? If there's no justification, then it probably isn't useful to include the option at all. If there is a justification, then it'd be helpful to include it in the answer. (Also, I don't like the example you've added, since it doesn't make clear that the argument to .match() is a regex; as written this lays a trap for the reader.)

@dimpiax 2019-04-17 11:07:53

@MarkAmery, you're right, in the current case – no reason to provide the example with function that accepts regex.

@wz366 2017-07-05 22:26:38

Another alternative is KMP.

The KMP algorithm provides worst-case linear time substring search, so it is a reasonable way if you do care about worst-case time complexity.

Here's a JavaScript implementation by Project Nayuki, taken from

// Searches for the given pattern string in the given text string using the Knuth-Morris-Pratt string matching algorithm.
// If the pattern is found, this returns the index of the start of the earliest match in 'text'. Otherwise -1 is returned.
function kmpSearch(pattern, text) {
    if (pattern.length == 0)
        return 0;  // Immediate match

    // Compute longest suffix-prefix table
    var lsp = [0];  // Base case
    for (var i = 1; i < pattern.length; i++) {
        var j = lsp[i - 1];  // Start by assuming we're extending the previous LSP
        while (j > 0 && pattern.charAt(i) != pattern.charAt(j))
            j = lsp[j - 1];
        if (pattern.charAt(i) == pattern.charAt(j))

    // Walk through text string
    var j = 0;  // Number of chars matched in pattern
    for (var i = 0; i < text.length; i++) {
        while (j > 0 && text.charAt(i) != pattern.charAt(j))
            j = lsp[j - 1];  // Fall back in the pattern
        if (text.charAt(i) == pattern.charAt(j)) {
            j++;  // Next char matched, increment position
            if (j == pattern.length)
                return i - (j - 1);
    return -1;  // Not found

Example usage:

kmpSearch('ays', 'haystack') != -1 // true
kmpSearch('asdf', 'haystack') != -1 // false

@eliocs 2013-01-07 10:23:16

There is a String.prototype.includes in ES6:

> true

Note that this does not work in Internet Explorer or some other old browsers with no or incomplete ES6 support. To make it work in old browsers, you may wish to use a transpiler like Babel, a shim library like es6-shim, or this polyfill from MDN:

if (!String.prototype.includes) {
  String.prototype.includes = function(search, start) {
    'use strict';
    if (typeof start !== 'number') {
      start = 0;

    if (start + search.length > this.length) {
      return false;
    } else {
      return this.indexOf(search, start) !== -1;

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