Question

Possible Duplicate:
Javascript – array.contains(obj)

What’s wrong with this:

var zipCodes =(['90001','90002','90003']);

Test if the value exists in the array zipCodes

if('90001' in zipCodes) {
  alert('True');
};

Answer 1

The in operator looks at property names, not values.

Because it’s an Array, the property names will be the indices of the Array.

If you’re only supporting a modern environment, you could use Array.prototype.indexOf().

if(zipCodes.indexOf('90001') > -1) {

If you need to support environments that don’t have .indexOf(), you could implement the MDN fix.

if (!Array.prototype.indexOf) {
    Array.prototype.indexOf = function (searchElement /*, fromIndex */ ) {
        "use strict";

        if (this === void 0 || this === null) throw new TypeError();

        var t = Object(this);
        var len = t.length >>> 0;
        if (len === 0) return -1;

        var n = 0;
        if (arguments.length > 0) {
            n = Number(arguments[1]);
            if (n !== n) // shortcut for verifying if it's NaN
            n = 0;
            else if (n !== 0 && n !== (1 / 0) && n !== -(1 / 0)) n = (n > 0 || -1) * Math.floor(Math.abs(n));
        }

        if (n >= len) return -1;

        var k = n >= 0 ? n : Math.max(len - Math.abs(n), 0);

        for (; k < len; k++) {
            if (k in t && t[k] === searchElement) return k;
        }
        return -1;
    };
}