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?

3 comments

@Fabien Ménager 2009-11-24 13:05:36

String#includes()

ES6 introduced String.prototype.includes:

var string = "foo",
    substring = "oo";

string.includes(substring)

includes doesn’t have IE support, though.

String#indexOf()

In an ES5 or older environments, String.prototype.indexOf returns the index of a substring (or -1 if not found):

var string = "foo",
    substring = "oo";

string.indexOf(substring) !== -1

RegExp#test()

More advanced users may prefer RegExp#test, which allows for testing for against regular expressions:

var string = "foo",
    regex = /oo/;

regex.test(string);

@kgbph 2019-05-21 09:15:44

Not supporting IE is a feature though

@rob74 2019-05-24 11:18:20

I don't like IE either, but if you have two functions that are largely identical, and one is better supported than the other one, I think you should pick the better supported one? So indexOf() it is...

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

Another alternative is KMP.

The KMP algorithm searches for a length-m substring in a length-n string in worst-case O(n+m) time, compared to a worst case of O(nm) for the naive algorithm, so using KMP may be reasonable if you care about worst-case time complexity.

Here's a JavaScript implementation by Project Nayuki, taken from https://www.nayuki.io/res/knuth-morris-pratt-string-matching/kmp-string-matcher.js:

// 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))
            j++;
        lsp.push(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:

"potato".includes("to");
> 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;
    }
  };
}

@Derk Jan Speelman 2019-06-03 14:40:39

Just do "potato".includes("to"); and run it through Babel.

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