Error without new operator

Question

I was reading this example on Promise. I realized that Error constructor is not called with new operator. Why does it work, then?

Answer 1

Because the specification says that the constructor function should check how it is called, and if it isn't called with new it should call itself with new and return that.

The Error constructor is the %Error% intrinsic object and the initial value of the Error property of the global object. When Error is called as a function rather than as a constructor, it creates and initializes a new Error object. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.

An example implementation of that in JS might look like:

function MyC(foo, bar) {
  if (!(this instanceof MyC)) {
    return new MyC(foo, bar);
  }
  this.foo = foo;
  this.bar = bar;
}