How to create a function that can't be used as constructor

Question

I am trying to create a function in javascript which can't be called as constructor like lets say if function is function SomeFunction(){}, on calling it like new SomeFunction() it should throw an exception but calling it without new should work.

Is there a way to achieve this?

I am curious to understand if there is a way to know inside a function if it's called with new or not. — Gautam, Apr 09 '19 at 06:20
Yes, it's *possible* but it seems to be an XY problem. What is the real issue you want to solve? — VLAZ, Apr 09 '19 at 06:20
Possible duplicate of [How to detect if a function is called as constructor?](https://stackoverflow.com/questions/367768/how-to-detect-if-a-function-is-called-as-constructor) — adiga, Apr 09 '19 at 06:23

score 2 · Answer 1 · answered Apr 09 '19 at 06:20

Use new.target. This is a special property, which is set when the function is called as constructor. See the following example.

function test() {
  if (new.target) {
    throw new Error('Can not call as constructor');
  }
  
  return 'test';
}

console.log(test());
console.log(new test());

score 2 · Accepted Answer · answered Apr 09 '19 at 06:23

2

You can also use an es6 arrow => function for this, as they are not allowed as constructor functions.

let SomeFunction = ()=>{ console.log("called");}
SomeFunction();
//Error for arrow function
new SomeFunction();

answered Apr 09 '19 at 06:23

Fullstack Guy

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score 1 · Answer 3 · answered Apr 09 '19 at 06:32

The constructor call creates an instance, so you can check if an instance is not created.

function SomeFunction () {
 if(this instanceof SomeFunction)
  throw('constructor called');
 console.log('function called')
}

SomeFunction();

new SomeFunction()

How to create a function that can't be used as constructor

3 Answers3