Setting the prototype of a function without proto in Javascript

Question

I want to create a constructor that has an object as its prototype.

For example:

var constructor=function(){
  this.foo=bar;
}

var constructorProto={
  method:function(){}
}

constructor.__proto__=constructorProto;

constructor.method();
new constructor;

Functional demo: http://jsfiddle.net/juwt5o97/

This allows me to pass the constructor along and modify it before calling new. However, I don't want to use __proto__ or Object.setPrototypeOf(). Is there a "proper" way of doing this?

prototypes are not static methods, so it's confusing what you want... you can just say constructor.method=constructorProto.method;, or use Extend or Object.create() to copy. — dandavis
– dandavis, Commented Mar 19, 2015 at 20:53
I remember reading somewhere that it shouldn't be used, maybe I got it mixed up with __proto__. Regardless, it's ES6 and I'd prefer if there's a method that works with ES5. — Leo Jiang
– Leo Jiang, Commented Mar 19, 2015 at 20:54
"I remember reading somewhere that it shouldn't be used..." I think you probably missed the point. They're likely not saying that that particular way of changing the prototype shouldn't be used. They're likely saying you shouldn't change it at all. — Lye Fish
– Lye Fish, Commented Mar 19, 2015 at 20:56
@dandavis The constructor may be created multiple times, so I don't want to recreate the methods for each constructor. — Leo Jiang
– Leo Jiang, Commented Mar 19, 2015 at 20:57

bvaughn · Accepted Answer · 2015-03-19 20:59:10Z

1

If you want to extend the prototype of your first class (so that instances inherit the methods) you can do so with Object.create:

var ClassA=function(){
  this.foo='bar';
}

var protoObject = {
  method:function(){alert('t');}
}

ClassA.prototype = Object.create(protoObject);

new ClassA().method();

If you want to just attach static functions to the first function, then you can do it like this:

for (var property in protoObject) {
  if (typeof protoObject[property] == 'function') {
    ClassA[property] = protoObject[property];
  }
}

ClassA.method();

edited Mar 19, 2015 at 20:59

answered Mar 19, 2015 at 20:54

bvaughn

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7 Comments

bvaughn Over a year ago

This is allowing him "to modify it before calling new". He just can't invoke it before calling new. That would... not involve the prototype at all. It would just be a static property...

dandavis Over a year ago

but i think he want to run constructor.method() before an instance is created... it's not a clear question.

bvaughn Over a year ago

Eh, I'll update my example then, but if that's what he wants... he's misunderstanding how the prototype works

dandavis Over a year ago

i'm pretty sure that's the case, but i like how you handle both potentials.

bvaughn Over a year ago

Thanks. :) And thanks for pointing out that I may have misunderstood what he was looking for.

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