if.ts

import { IterableX } from '../../iterable';
import { _if as ifStatic } from '../../iterable/if';

function* _if<T>(
    fn: () => boolean,
    thenSource: Iterable<T>,
    elseSource: Iterable<T>) {
  return new IterableX(ifStatic(fn, thenSource, elseSource));
}

IterableX.if = _if;

Try this:

import { IterableX } from '../../iterable';
import { _if as ifStatic } from '../../iterable/if';

function* _if<T>(
    fn: () => boolean,
    thenSource: Iterable<T>,
    elseSource: Iterable<T>) {
    return new IterableX(ifStatic(fn, thenSource, elseSource));
}

IterableX.if = _if;

declare module "../../iterable" {
  namespace IterableX {
    function _if<T>(fn: () => boolean, thenSource: Iterable<T>, elseSource: Iterable<T>): IterableIterator<T>;
    export { _if as if }
  }
}