Dynamic instantiation in JavaScript

Question

Apart from eval(`o = new ${className}(${args.join(", ")})`), is there any other way to instantiate an object using variable argument list?

E.g.: var foo = instantiate(className, [arg1, arg2, ...])

Answer 1

You can instantiate an object with a variable argument list like this:

function instantiate(className, args) {
    var o, f, c;
    c = window[className]; // get reference to class constructor function
    f = function(){}; // dummy function
    f.prototype = c.prototype; // reference same prototype
    o = new f(); // instantiate dummy function to copy prototype properties
    c.apply(o, args); // call class constructor, supplying new object as context
    o.constructor = c; // assign correct constructor (not f)
    return o;
}

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Side note: you may wish to pass a direct reference to the class constructor function:

var foo = instantiate(Array, [arg1, arg2, ...]);
// Instead of:
var foo = instantiate("Array", [arg1, arg2, ...]);

... which makes this compatible with non-global functions.

Answer 2

Using Object.create() in ES5:

function instantiate(constructor, args) {
    var instance = Object.create(constructor.prototype);
    constructor.apply(instance, args);
    return instance;
}

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Using the spread operator in ES6:

var foo = new constructor(...args);