JavaScriptのArrayの拡張2

ArrayCustom2.js

//write kazua
 
if (!Array.prototype.contains) {
    Array.prototype.contains = function(value) {
        for ( var i = 0; i < this.length; i++)
            if (this[i] === value)
                return true;
        return false;
    };
}
if (!Array.prototype.distinct) {
    Array.prototype.distinct = function() {
        if (this.length == 0)
            return this;
        var na = [];
        for ( var i = 0; i < this.length; i++)
            if (!na.contains(this[i]))
                na.push(this[i]);
 
        return na;
    };
}
if (!Array.prototype.zip) {
    Array.prototype.zip = function(array) {
        var len = this.length > array.length ? array.length : this.length;
        if (len == 0)
            return [];
        var na = [];
        for ( var i = 0; i < len; i++)
            na.push([ this[i], array[i] ]);
 
        return na;
    };
}
if (!Array.prototype.zipWithIndex) {
    Array.prototype.zipWithIndex = function() {
        if (this.length == 0)
            return this;
        var na = [];
        for ( var i = 0; i < this.length; i++)
            na.push([ this[i], i ]);
 
        return na;
    };
}
if (!Array.prototype.head) {
    Array.prototype.head = function() {
        return this.slice(0, 1);
    };
}
if (!Array.prototype.tail) {
    Array.prototype.tail = function() {
        return this.slice(1);
    };
}
if (!Array.prototype.last) {
    Array.prototype.last = function() {
        return this.slice(this.length - 1, this.length);
    };
}
if (!Array.prototype.init) {
    Array.prototype.init = function() {
        return this.slice(0, this.length - 1);
    };
}