JavaScript Array extensions

function id(x) { return x; }

Array.prototype.where = function(predicate) {
    var newArray=[];
    for (var i=0;i<this.length;i++) {
        var item=this[i];
        if(predicate(item)) newArray.push(item);
    }
    return newArray;
}

Array.prototype.choose = function(chooser) {
    var newArray=[];
    for (var i=0;i<this.length;i++) {
        var item=chooser(this[i]);
        if(item!=null) newArray.push(item);
    }
    return newArray;
}

Array.prototype.countBy=function(getKey) {
    if(getKey==undefined) getKey=id;
    var counts={};
    for(var i=0;i<this.length;i++) {
        var key=getKey(this[i]);
        var count=counts[key];
        counts[key]=(count==undefined)?1:count+1;
    }
    return counts;
}