How do I check if an array includes a value in JavaScript?

Question

What is the most concise and efficient way to find out if a JavaScript array contains a value?

This is the only way I know to do it:

function contains(a, obj) {     for (var i = 0; i < a.length; i++) {         if (a[i] === obj) {             return true;         }     }     return false; } 

Is there a better and more concise way to accomplish this?

Answer 1

Modern browsers have Array#includes, which does exactly that and is widely supported by everyone except IE:

console.log(['joe', 'jane', 'mary'].includes('jane')); //true

You can also use Array#indexOf, which is less direct, but doesn't require polyfills for outdated browsers.

console.log(['joe', 'jane', 'mary'].indexOf('jane') >= 0); //true

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Many frameworks also offer similar methods:

• jQuery: $.inArray(value, array, [fromIndex])
• Underscore.js: _.contains(array, value) (also aliased as _.include and _.includes)
• Dojo Toolkit: dojo.indexOf(array, value, [fromIndex, findLast])
• Prototype: array.indexOf(value)
• MooTools: array.indexOf(value)
• MochiKit: findValue(array, value)
• MS Ajax: array.indexOf(value)
• Ext: Ext.Array.contains(array, value)
• Lodash: _.includes(array, value, [from]) (is _.contains prior 4.0.0)
• Ramda: R.includes(value, array)

Notice that some frameworks implement this as a function, while others add the function to the array prototype.

Answer 2

Update from 2019: This answer is from 2008 (11 years old!) and is not relevant for modern JS usage. The promised performance improvement was based on a benchmark done in browsers of that time. It might not be relevant to modern JS execution contexts. If you need an easy solution, look for other answers. If you need the best performance, benchmark for yourself in the relevant execution environments.

As others have said, the iteration through the array is probably the best way, but it has been proven that a decreasing while loop is the fastest way to iterate in JavaScript. So you may want to rewrite your code as follows:

function contains(a, obj) {     var i = a.length;     while (i--) {        if (a[i] === obj) {            return true;        }     }     return false; } 

Of course, you may as well extend Array prototype:

Array.prototype.contains = function(obj) {     var i = this.length;     while (i--) {         if (this[i] === obj) {             return true;         }     }     return false; } 

And now you can simply use the following:

alert([1, 2, 3].contains(2)); // => true alert([1, 2, 3].contains('2')); // => false