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

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?

This is very closely related to question Best way to find an item in a JavaScript Array? which addresses finding objects in an array using indexOf.

Answer

Current browsers have Array#includes, which does exactly that, is widely supported, and has a polyfill for older browsers.

> ['joe', 'jane', 'mary'].includes('jane');
true 

You can also use Array#indexOf, which is less direct, but doesn't require Polyfills for out of date browsers.

jQuery offers $.inArray, which is functionally equivalent to Array#indexOf.

underscore.js, a JavaScript utility library, offers _.contains(list, value), alias _.include(list, value), both of which use indexOf internally if passed a JavaScript array.

Some other frameworks offer similar methods:

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