Question

So I’m looking at some code from “Eloquent Javascript” and it says that to cause inheritance you might write a function like:

function clone(object) {
    function OneShotConstructor() {}
    OneShotConstructor.prototype = object;
    return new OneShotConstructor();
}

and would call it like so, if a class Terrarium was already defined:

function LifeLikeTerrarium(plan) {
    Terrarium.call(this, plan);
}
LifeLikeTerrarium.prototype = clone(Terrarium.prototype);
LifeLikeTerrarium.prototype.constructor;

So my question is why we need to create the OneShotConstructor in order to copy its prototype. Why couldn’t we just have written something like:

LifeLikeTerrarium.prototype = new Terrarium();

Would this create any problems, is it improper javascript?

Answer 1

See Crockford’s prototypal inheritance:

http://javascript.crockford.com/prototypal.html

beyond setting the prototype, you need to call the new keyword which sets the object’s internal [[prototype]] property to be the constructor function’s external prototype. It then executes the constructor function, using the new object whenever this is mentioned..

A nice explanation is here:

What is the 'new' keyword in JavaScript?

and here:

http://joost.zeekat.nl/constructors-considered-mildly-confusing.html