How to extend the Generator class?

Question

I tried to filter a generator and had the expectation that this kind of general functionality must be defined anywhere in JavaScript, because it is defined for Arrays, but I can not find it. So I tried to define it. But I can not extend the built-in generators.

I have an example generator

function make_nums ()
{
  let nums = {};
  nums[Symbol.iterator] = function* () {
    yield 1;
    yield 2;
    yield 3;
  };
  return nums;
}

generating some numbers.

[...make_nums()] // => Array [ 1, 2, 3 ]

If I build an array, I can filter the array by the use of the filter function for arrays.

[...make_nums()].filter(n => n > 1) // => Array [ 2, 3 ]

But I do not want to build an array. Instead I want to take the old generator and build a new filtering generator. For this I wrote the following function.

function filtered (generator, filter)
{
  let g = {};
  g[Symbol.iterator] = function* () {
    for (let value of generator)
      if (filter(value))
        yield value;
  };
  return g;
}

which can be used to do what I want.

[...filtered (make_nums(), n => n > 1)] // => Array [ 2, 3 ]

But this is a very general function, which can be applied to every generator in the same way the filter function can be applied to every Array. So I tried to extend generators in general, but I do not understand how.

The MDN documentation for generators suggests somehow that Generator.prototype may exist, but it does not seem to exist. When I try to define something in Generator.prototype, I get the error

ReferenceError: Generator is not defined

How can I extend the built-in Generator class?

How to extend the Generator class?

2 Answers