Lately I have been doing a lot work with passing data back and forth between Javascript and PHP and I often am working on arrays on the Javascript side and there is no easy native method to test whether an array contains a certain value. So I wrote my own.

function in_array(needle, haystack, strict) {
    for(var i = 0; i < haystack.length; i++) {
        if(strict) {
            if(haystack[i] === needle) {
                return true;
            }
        } else {
            if(haystack[i] == needle) {
                return true;
            }
        }
    }

    return false;
}

You could also prototype it to be built in to the native Array object. You just have to tweak the code a little.

function in_array(needle, strict) {
    for(var i = 0; i < this.length; i++) {
        if(strict) {
            if(this[i] === needle) {
                return true;
            }
        } else {
            if(this[i] == needle) {
                return true;
            }
        }
    }

    return false;
}

Array.prototype.in_array = in_array;

// example usage

var test = new Array();
test.push('i');
test.push(1);

test.in_array('i'); // true
test.in_array('i', true); // true
test.in_array('b'); // false
test.in_array(1, true); // true
test.in_array('1', true); // false