javascript apply on constructor, throwing “malformed formal parameter”

Question

thanks to wonderful responses to this question I understand how to call javascript functions with varargs.

now I\'m looking to use apply with a constructor

Answer 1

You can exploit the fact that you can chain constructors using apply(...) to achieve this, although this requires the creation of a proxy class. The construct() function below lets you do:

var f1 = construct(Foo, [2, 3]);
// which is more or less equivalent to
var f2 = new Foo(2, 3);

The construct() function:

function construct(klass, args) {

  function F() {
    return klass.apply(this, arguments[0]); 
  }; 

  F.prototype = klass.prototype; 

  return new F(args);

}

Some sample code that uses it:

function Foo(a, b) {
  this.a = a; this.b = b;
}

Foo.prototype.dump = function() {
  console.log("a = ", this.a);
  console.log("b = ", this.b);
};

var f = construct(Foo, [7, 9]);

f.dump();

Answer 2

You could use apply and pass an empty object as the this argument:

var mid_parser = {};
Parser.apply(mid_parser, mid_patterns);

But that solution will not take care about the prototype chain.

You could create a Parser object, using the new operator, but without passing arguments, and then use apply to re-run the constructor function:

var mid_parser = new Parser();
Parser.apply(mid_parser, mid_patterns);

Answer 3

To complete @CMS solution and preserve the prototype chain, you can do this:

var mid_parser = {};
mid_parser.__proto__ = Parser.prototype;
Parser.apply(mid_parser, mid_patterns);

As a side note, it will not work with IE 8-.

Answer 4

A better solution is to create a temporary constructor function, apply the prototype of the class that you want (to ensure prototype chains are preserved) and then apply the constructor manually. This prevents calling the constructor twice unnecessarily...

applySecond = function(){
    function tempCtor() {};
    return function(ctor, args){
        tempCtor.prototype = ctor.prototype;
        var instance = new tempCtor();
        ctor.apply(instance,args);
        return instance;
    }
}();

I tested the performance and found that this method is, in fact, a bit slower in the very simple case. However, it only takes the construction of a single Date() object in the constructor for this to be more efficient. Also, don't forget that some constructors may throw exceptions if there are no parameters passed, so this is also more correct.

My validation code:

var ExpensiveClass = function(arg0,arg1){
    this.arg0 = arg0;
    this.arg1 = arg1;
    this.dat = new Date();
}

var CheapClass = function(arg0,arg1){
    this.arg0 = arg0;
    this.arg1 = arg1;
}

applyFirst = function(ctor, args){
    var instance = new ctor();
    ctor.apply(instance, args);
    return instance;
}

applySecond = function(){
    function tempCtor() {};
    return function(ctor, args){
        tempCtor.prototype = ctor.prototype;
        var instance = new tempCtor();
        ctor.apply(instance,args);
        return instance;
    }
}();

console.time('first Expensive');
for(var i = 0; i < 10000; i++){
    test = applyFirst(ExpensiveClass ,['arg0','arg1']);
}
console.timeEnd('first Expensive');

console.time('second Expensive');
for(var i = 0; i < 10000; i++){
    test = applySecond(ExpensiveClass ,['arg0','arg1']);
}
console.timeEnd('second Expensive');

console.time('first Cheap');
for(var i = 0; i < 10000; i++){
    test = applyFirst(CheapClass,['arg0','arg1']);
}
console.timeEnd('first Cheap');

console.time('second Cheap');
for(var i = 0; i < 10000; i++){
    test = applySecond(CheapClass,['arg0','arg1']);
}
console.timeEnd('second Cheap');

The results:

first Expensive: 76ms
second Expensive: 66ms
first Cheap: 52ms
second Cheap: 52ms