Javascript ES6, why I can not use `new` with arrow function? [duplicate]

Question

As far as I know, the arrow function is similar to a normal function. There aren’t any problem when I use it like this:

let X = () => {};
let Y = function() {};
X();
Y();

However, the error occurred when I used them with new:

let X = () => {};
let Y = function() {};
x = new X();
y = new Y();

Uncaught TypeError: X is not a constructor

Why is that?

Answer 1

Q. What did I do wrong?

A. You used new with an arrow function, and that's not allowed.

Q. Can I turn an arrow function into a constructor?

A. Only by wrapping it in a normal function, which would be silly. You can't turn an arrow function itself into a constructor.

Q. Can you explain how the specification disallows new with arrow functions?

A. To be a constructor, a function object must have a [[Construct]] internal method.

Functions created with the function keyword are constructors, as are some built-in functions such as Date. These are the functions you can use with new.

Other function objects do not have a [[Construct]] internal method. These include arrow functions. So you can't use new with these. This makes sense since you can't set the this value of an arrow function.

Some built-in functions are also not constructors. E.g. you can't do new parseInt().

Q. Can you explain the rationale behind disallowing new with arrow functions in the specification?

A. Use common sense, or search the es-discuss archives.