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?