Version-dependent type inheritance in Node.js

Question

Why does the following code work only under Node.js 5.x and 6.x, but breaks in 4.x and earlier versions?

Is there a way to modify it in such a way to make work in any Node.js 0.10.x - 6.x?

'use strict';

var util = require('util');

function Parent() {
}

Parent.prototype.show = function () {
    return this.msg(); // virtual-like call;
};

function Child() {
}

Child.prototype.msg = function () {
    return 'Hello!';
};

util.inherits(Child, Parent);

var test = new Child();

console.log(test.show());

In Node.js 5.x and 6.x it displays Hello!. And in any earlier version of Node.js it displays TypeError: this.msg is not a function.

Steve Kinney · Accepted Answer · 2016-05-03 13:17:03Z

3

Prior to Node 5.0, util.inherits used Object.create() to create the inheritance chain. This unfortunately had a bug. Anything you attached to Child.prototype prior to using util.inherits was removed—which is what is causing your bug. As of 5.0 and later, Node uses Object.setPrototypeOf under the hood.

Luckily, the fix is super simple. Move the util.inherits call before you add methods to Child.prototype.

The following code will work from 0.10 to 6.0.

'use strict';

var util = require('util');

function Parent() {
}

Parent.prototype.show = function () {
  return this.msg(); // virtual-like call;
};

function Child() {
}

util.inherits(Child, Parent);

Child.prototype.msg = function () {
  return 'Hello!';
};

var test = new Child();

console.log(test.show());

answered May 3, 2016 at 13:17

Steve Kinney

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6 Comments

vitaly-t Over a year ago

Great answer, right to the point! That little trick did it! :)

vitaly-t Over a year ago

After trying to use it I found it not usable across several modules. It can only work within just one JavaScript file, which is hardly usable. I found it that using the polyfill for setPrototypeOf (earlier answer by @Blaze) is a far better solution.

Steve Kinney Over a year ago

Can you give me an example where it's not usable?

vitaly-t Over a year ago

I have classes Parent, ChildA, ChildB - all sitting in separate modules. The only way to correct the sequence of prototyping is by moving them all into a single module. It is not possible otherwise. I would have to have them all declared first, then do the inheritance, and only then add all the prototype methods. Such mess requires that it is all in one file.

Steve Kinney Over a year ago

Not necessarily! All that has to happens is that your util.inherits() calls need to be before you add anything to the prototype.

|

Soubhik Mondal · Accepted Answer · 2016-05-03 13:16:22Z

1

Taking a look at the two different versions of code I found that the only difference between them is the way in which prototypical inheritance is achieved.

4.x

ctor.prototype = Object.create(superCtor.prototype, {
    constructor: {
      value: ctor,
      enumerable: false,
      writable: true,
      configurable: true
    }
});

Where ctor is the child constructor and superCtor is the parent constructor.

Master

Object.setPrototypeOf(ctor.prototype, superCtor.prototype);

Where ctor is the child constructor and superCtor is the parent constructor.

For versions 4.x (and below probably), the parent's prototype is assigned to the child's prototype, thus, completely replacing it.

However, the latest version uses Object.setPrototypeOf which I'm assuming doesn't replace the child's prototype but preserves both the child's prototype as well as parent's prototype.

Update: I found this in the ECMAScript 2017 spec, if it's of any help...

edited May 3, 2016 at 13:16

answered May 3, 2016 at 13:08

Soubhik Mondal

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3 Comments

vitaly-t Over a year ago

Unfortunately, setPrototypeOf doesn't exist in Node.js 0.10.x. Any idea how to work-around this?

Soubhik Mondal Over a year ago

You can use the polyfill for Object.setPrototypeOf or make your own custom inherits function.

vitaly-t Over a year ago

Using the polyfill for setPrototypeOf is the best solution. Thank you!

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