How to check whether a string contains a substring in JavaScript?

ECMAScript 6 introduced String.prototype.includes:

const string = "foo"; const substring = "oo";  console.log(string.includes(substring)); // true

includes doesn’t have Internet Explorer support, though. In ECMAScript 5 or older environments, use String.prototype.indexOf, which returns -1 when a substring cannot be found:

var string = "foo"; var substring = "oo";  console.log(string.indexOf(substring) !== -1); // true

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;     }   }; } 

Another alternative is KMP (Knuth–Morris–Pratt).

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(n⋅m) 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[i] !== pattern[j])       j = lsp[j - 1];     if (pattern[i] === pattern[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[i] != pattern[j])       j = lsp[j - 1]; // Fall back in the pattern     if (text[i]  == pattern[j]) {       j++; // Next char matched, increment position       if (j == pattern.length)         return i - (j - 1);     }   }   return -1; // Not found }  console.log(kmpSearch('ays', 'haystack') != -1) // true console.log(kmpSearch('asdf', 'haystack') != -1) // false