How do I check if an array includes an object in JavaScript?

What is the most concise and efficient way to find out if a JavaScript array contains an object?
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 Stack Overflow question Best way to find an item in a JavaScript Array? which addresses finding objects in an array using indexOf.


Answer : 

Modern browsers have Array#indexOf, which can do that. Even newer browsers have Array#includes, which does exactly that, but unfortunately it's not widely supported. Older browsers can be supported using this polyfill.
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.

Languages that compile to JavaScript

In CoffeeScript, the in operator is the equivalent of contains:
a = [1, 2, 3, 4]
alert(2 in a)
var mylist = [1, 2, 3];
assert(mylist.contains(1));
assert(mylist.indexOf(1) == 0);



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