var arr=["a","b"];

alert(arr.in_array("a"))

Array.prototype.contains = function(obj) {

    var i = this.length;

    while (i--) {

        if (this[i] === obj) {

            return true;

        }

    }

    return false;

}

或

Array.prototype.contains = function(element) {

    for (var i = 0; i < this.length; i++) {

        if (this[i] == element) {

            return true;

        }

    }

    return false;

}

或

Array.prototype.in_array = function(e) {

    for(i=0; i<this.length && this[i]!=e; i++);

    return !(i==this.length);

}

还有一个大牛是这样写的：

Array.prototype.S = String.fromCharCode(2);

Array.prototype.in_array = function(e) {

    var r = new RegExp(this.S+e+this.S);

    return (r.test(this.S+this.join(this.S)+this.S));

}