How can I call a javascript constructor using call or apply? [duplicate]

Question

This is how you do it:

function applyToConstructor(constructor, argArray) {
    var args = [null].concat(argArray);
    var factoryFunction = constructor.bind.apply(constructor, args);
    return new factoryFunction();
}

var d = applyToConstructor(Date, [2008, 10, 8, 00, 16, 34, 254]);

Call is slightly easier

function callConstructor(constructor) {
    var factoryFunction = constructor.bind.apply(constructor, arguments);
    return new factoryFunction();
}

var d = callConstructor(Date, 2008, 10, 8, 00, 16, 34, 254);

You can use either of these to create factory functions:

var dateFactory = applyToConstructor.bind(null, Date)
var d = dateFactory([2008, 10, 8, 00, 16, 34, 254]);

or

var dateFactory = callConstructor.bind(null, Date)
var d = dateFactory(2008, 10, 8, 00, 16, 34, 254);

It will work with any constructor, not just built-ins or constructors that can double as functions (like Date).

However it does require the Ecmascript 5 .bind function. Shims will probably not work correctly.

A different approach, more in the style of some of the other answers is to create a function version of the built in new. This will not work on all builtins (like Date).

function neu(constructor) {
    // http://www.ecma-international.org/ecma-262/5.1/#sec-13.2.2
    var instance = Object.create(constructor.prototype);
    var result = constructor.apply(instance, Array.prototype.slice.call(arguments, 1));

    // The ECMAScript language types are Undefined, Null, Boolean, String, Number, and Object.
    return (result !== null && typeof result === 'object') ? result : instance;
}

function Person(first, last) {this.first = first;this.last = last};
Person.prototype.hi = function(){console.log(this.first, this.last);};

var p = neu(Person, "Neo", "Anderson");

And now, of course you can do .apply or .call or .bind on neu as normal.

For example:

var personFactory = neu.bind(null, Person);
var d = personFactory("Harry", "Potter");

I feel that the first solution I give is better though, as it doesn’t depend on you correctly replicating the semantics of a builtin and it works correctly with builtins.

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