How can I call a javascript constructor using call or apply?

Question

How could I generalise the function below to take N arguments? (Using call or apply?)

Is there a programmatic way to apply arguments to 'new'? I don't want the constructor to be treated like a plain function.

/**
 * This higher level function takes a constructor and arguments
 * and returns a function, which when called will return the 
 * lazily constructed value.
 * 
 * All the arguments, except the first are pased to the constructor.
 * 
 * @param {Function} constructor
 */ 

function conthunktor(Constructor) {
    var args = Array.prototype.slice.call(arguments, 1);
    return function() {
        console.log(args);
        if (args.length === 0) {
            return new Constructor();
        }
        if (args.length === 1) {
            return new Constructor(args[0]);
        }
        if (args.length === 2) {
            return new Constructor(args[0], args[1]);
        }
        if (args.length === 3) {
            return new Constructor(args[0], args[1], args[2]);
        }
        throw("too many arguments");    
    }
}

qUnit test:

test("conthunktorTest", function() {
    function MyConstructor(arg0, arg1) {
        this.arg0 = arg0;
        this.arg1 = arg1;
    }
    MyConstructor.prototype.toString = function() {
        return this.arg0 + " " + this.arg1;
    }

    var thunk = conthunktor(MyConstructor, "hello", "world");
    var my_object = thunk();
    deepEqual(my_object.toString(), "hello world");
});

Answer 1

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.

Answer 2

Try this:

function conthunktor(Constructor) {
    var args = Array.prototype.slice.call(arguments, 1);
    return function() {

         var Temp = function(){}, // temporary constructor
             inst, ret; // other vars

         // Give the Temp constructor the Constructor's prototype
         Temp.prototype = Constructor.prototype;

         // Create a new instance
         inst = new Temp;

         // Call the original Constructor with the temp
         // instance as its context (i.e. its 'this' value)
         ret = Constructor.apply(inst, args);

         // If an object has been returned then return it otherwise
         // return the original instance.
         // (consistent with behaviour of the new operator)
         return Object(ret) === ret ? ret : inst;

    }
}

Answer 3

This function is identical to new in all cases. It will probably be significantly slower than 999’s answer, though, so use it only if you really need it.

function applyConstructor(ctor, args) {
    var a = [];
    for (var i = 0; i < args.length; i++)
        a[i] = 'args[' + i + ']';
    return eval('new ctor(' + a.join() + ')');
}

UPDATE: Once ES6 support is widespread, you'll be able to write this:

function applyConstructor(ctor, args) {
    return new ctor(...args);
}

...but you won't need to, because the standard library function Reflect.construct() does exactly what you're looking for!

Answer 4

In ECMAScript 6, you can use the spread operator to apply a constructor with the new keyword to an array of arguments:

var dateFields = [2014, 09, 20, 19, 31, 59, 999];
var date = new Date(...dateFields);
console.log(date);  // Date 2014-10-20T15:01:59.999Z

Answer 5

Another approach, which requires to modify the actual constructor being called, but seems cleaner to me than using eval() or introducing a new dummy function in the construction chain... Keep your conthunktor function like

function conthunktor(Constructor) {
  // Call the constructor
  return Constructor.apply(null, Array.prototype.slice.call(arguments, 1));
}

And modify the constructors being called...

function MyConstructor(a, b, c) {
  if(!(this instanceof MyConstructor)) {
    return new MyConstructor(a, b, c);
  }
  this.a = a;
  this.b = b;
  this.c = c;
  // The rest of your constructor...
}

So you can try:

var myInstance = conthunktor(MyConstructor, 1, 2, 3);

var sum = myInstance.a + myInstance.b + myInstance.c; // sum is 6

Answer 6

Using a temporary constructor seems to be the best solution if Object.create is not available.

If Object.create is available, then using it is a much better option. On Node.js, using Object.create results in much faster code. Here's an example of how Object.create can be used:

function applyToConstructor(ctor, args) {
    var new_obj = Object.create(ctor.prototype);
    var ctor_ret = ctor.apply(new_obj, args);

    // Some constructors return a value; make sure to use it!
    return ctor_ret !== undefined ? ctor_ret: new_obj;
}

(Obviously, the args argument is a list of arguments to apply.)

I had a piece of code that originally used eval to read a piece of data created by another tool. (Yes, eval is evil.) This would instantiate a tree of hundreds to thousands of elements. Basically, the JavaScript engine was responsible for parsing and executing a bunch of new ...(...) expressions. I converted my system to parse a JSON structure, which means I have to have my code determine which constructor to call for each type of object in the tree. When I ran the new code in my test suite, I was surprised to see a dramatic slow down relative to the eval version.

1. Test suite with eval version: 1 second.
2. Test suite with JSON version, using temporary constructor: 5 seconds.
3. Test suite with JSON version, using Object.create: 1 second.

The test suite creates multiple trees. I calculated my applytoConstructor function was called about 125,000 times when the test suite is run.

Answer 7

There is a rehusable solution for this case. For every Class to you wish to call with apply or call method, you must call before to convertToAllowApply('classNameInString'); the Class must be in the same Scoope o global scoope (I don't try sending ns.className for example...)

There is the code:

function convertToAllowApply(kName){
    var n = '\n', t = '\t';
    var scrit = 
        'var oldKlass = ' + kName + ';' + n +
        kName + '.prototype.__Creates__ = oldKlass;' + n +

        kName + ' = function(){' + n +
            t + 'if(!(this instanceof ' + kName + ')){'+ n +
                t + t + 'obj = new ' + kName + ';'+ n +
                t + t + kName + '.prototype.__Creates__.apply(obj, arguments);'+ n +
                t + t + 'return obj;' + n +
            t + '}' + n +
        '}' + n +
        kName + '.prototype = oldKlass.prototype;';

    var convert = new Function(scrit);

    convert();
}

// USE CASE:

myKlass = function(){
    this.data = Array.prototype.slice.call(arguments,0);
    console.log('this: ', this);
}

myKlass.prototype.prop = 'myName is myKlass';
myKlass.prototype.method = function(){
    console.log(this);
}

convertToAllowApply('myKlass');

var t1 = myKlass.apply(null, [1,2,3]);
console.log('t1 is: ', t1);