Calling class instance as a function in JavaScript

Question

Assuming I have a class

class foo {
  constructor() {
    this._pos = 0;
  }
  
  bar(arg) {
    console.log(arg);
  }
}

const obj = new foo();

How do I make it possible to call:

let var1 = obj('something');

You can't "call" a plain object; what would that even mean? What code would run? — Pointy
– Pointy, Commented Mar 14, 2018 at 14:02
Do you want to call bar method of foo class or what? Please clarify. — standby954
– standby954, Commented Mar 14, 2018 at 14:02
What you have and what you want to do don't go with each other. You have an instance of a class (an object) and you want to invoke the object as a function. — Scott Marcus
– Scott Marcus, Commented Mar 14, 2018 at 14:03
meta.stackexchange.com/questions/66377/what-is-the-xy-problem — Yury Tarabanko
– Yury Tarabanko, Commented Mar 14, 2018 at 14:04
Technically you could extend built-in Function constructor using class Foo extends Function. Demo But you can doesn't mean you should. :) — Yury Tarabanko
– Yury Tarabanko, Commented Mar 14, 2018 at 14:09

user9366559 · Accepted Answer · 2018-03-14 14:30:51Z

9

You can make a callable object by extending the Function constructor, though if you want it to access the instance created, you'll actually need to create a bound function in the constructor that binds the instance to a function that is returned.

class foo extends Function {
  constructor() {
    super("...args", "return this.bar(...args)");
    this._pos = 0;
    return this.bind(this);
  }
  
  bar(arg) {
    console.log(arg + this._pos);
  }
}

const obj = new foo();

let var1 = obj('something ');

answered Mar 14, 2018 at 14:30

user9366559

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1 Comment

intellion Over a year ago

A very elegant solution! Thank you!

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