JavaScript Generator Utility Functions

README.md

This source file modifies the prototype of Generator Function objects to provide the following functions already available to arrays, allowing them to operate on demand on generators:

• map

• reduce

• join

• forEach

• concat

• entries

• every

• fill

• filter

• find

• push

• some

It also provides a range(start, end) generator function.

Note that unlike the original Array functions, there is no thisArg accepted, and callback functions will not receive a reference to the existing array as there is no array, only a generator.

gen.js

const GenConstructor = (function*(){}()).constructor;

GenConstructor.prototype.map = function*(alt) {
  let i = 0;
  for (let v of this) {
    yield alt(v, i++);
  }
}

GenConstructor.prototype.reduce = function(reducer, initialValue) {
  let i = 0;
  
  if (initialValue === undefined) {
    initialValue = this.next().value;
    i += 1;
  }
  
  for (let v of this) {
    initialValue = reducer(initialValue, v, i++);
  }
  
  return initialValue;
}

GenConstructor.prototype.join = function(joiner) {
  return this.reduce((a, b) => a.toString() + joiner + b.toString());
}

GenConstructor.prototype.forEach = function(looper) {
  let i = 0;
  
  for (let v of this) {
    looper(v, i++);
  }
}

function* range(a, b) {
  for (let v = a; v <= b; v++) {
    yield v;
  }
}

GenConstructor.prototype.concat = function*(other) {
  yield* this;
  yield* other;
}

GenConstructor.prototype.entries = function*() {
  yield* this.map((a, i) => [i, a]);
}

GenConstructor.prototype.every = function(test) {
  let i = 0;
  for (let v of this) {
    if (!test(v, i++)) return false;
  }
  
  return true;
}

GenConstructor.prototype.fill = function*(value, start, end) {  
  yield* this.map((a, i) => ((start === undefined || i >= start) && (end === undefined || i <= end)) ? value : a);
}

GenConstructor.prototype.filter = function*(callback) {
  let i = 0;
  for (let v of this) {
    if (callback(v, i++)) yield v;
  }
}

GenConstructor.prototype.find = function(callback) {
  return this.filter(callback).next().value;
}

GenConstructor.prototype.push = function() {
  yield* this;
  yield* arguments;
}

GenConstructor.prototype.some = function(test) {
  return this.every((a, i) => !test(a, i));
}