CODEWITHSUNDEEP
node.jstransform-stream
krave
krave

Reputation: 1919

Two confusions about transform stream part of node.js documentation around pre-ES6 style

Here is the doc I am confused with.

When using pre-ES6 style constructors

const { Transform } = require('stream');
const util = require('util');

function MyTransform(options) {
  if (!(this instanceof MyTransform))
    return new MyTransform(options);
  Transform.call(this, options);
}
util.inherits(MyTransform, Transform);

  1. Why do we need to check this instanceof MyTransform? As far as I know, as long as we invoke new MyTransform(), evaluation of this instanceof MyTransfrom will always return true. Maybe using MyTransform() to create a Transform instance can be found in many code bases? This is the only reason I could guess.

  2. What is the purpose of util.inherits(MyTransform, Transform); ? Just to ensure that new MyTransform() instanceof Transform returns true?

Thank you for your time in advance!

Upvotes: 0

Views: 80

Answers (1)

Mike 'Pomax' Kamermans
Mike 'Pomax' Kamermans

Reputation: 53705

  1. MyTransform is just a function like any other function. There's nothing special about it, and so calling it as a function (without new) will work perfectly fine. So what should happen in that case? According to this code: fall through to a constructor call and returning the resulting instance object.
  2. As per the documentation for that function it enforces prototype inheritance, because again, you wrote MyTransform as just a plain function: while you can use new with any function, you didn't write any of the code necessary for proper prototype inheritance so using new will give you a completely useless object. That means either you add the code necessary to set set up prototype inheritance yourself, or you ask a utility function to do that for you.

Upvotes: 1

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