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Can You Call a Class Constructor Without \'new\' in ES6?

Oct 26, 2024 am 01:55 AM

 Can You Call a Class Constructor Without 'new' in ES6?

Call Class Constructor Without new Keyword in ES6

Given the class definition:

class Foo {
  constructor(x) {
    if (!(this instanceof Foo)) return new Foo(x);
    this.x = x;
  }
  hello() {
    return `hello ${this.x}`;
  }
}
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It is not possible to directly call the class constructor without the new keyword. This is because classes in ES6 inherently have a constructor function that is invoked when the class is called.

Invoking a class without new results in an error:

Cannot call a class as a function
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This error message clearly indicates that the class constructor can only be called with the new operator, which is required to create a new instance of the class.

To overcome this limitation, consider the following approaches:

  • Use a regular function instead:
function Foo(x) {
  this.x = x;
  this.hello = function() {
    return `hello ${this.x}`;
  }
}
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  • Always call the class with new:
(new Foo("world")).hello(); // "hello world"
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  • Wrap the class in a function and call with new:
var FooWrapper = function(...args) { return new Foo(...args) };
FooWrapper("world").hello(); // "hello world"
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