JavaScript: Folding infinite streams (generator function)

Question

In Java it is possible to declare and fold infinite streams as so

List<Integer> collect = Stream.iterate(0, i -> i + 2)
    .map(i -> i * 3)
    .filter(i -> i % 2 == 0)
    .limit(10)
    .collect(Collectors.toList());

// -> [0, 6, 12, 18, 24]

In JavaScript I could use generator functions to yield and spread stream of values.

// Limit the value in generator
let generator = (function* () {
    for (let i=0; i<10; i++) {
        yield i
    }
})()

[ ...generator ]
    .map(i => i * 3)
    .filter(i => i % 2 === 0)

// -> [0, 6, 12, 18, 24]

But how could I stream and fold an infinite stream? I know I could iterate and limit the stream with for (n of generator) loop. But is it possible with fluent API such as Java example?

Answer 1

Here is an alternative approach to the given answer.

1. Functional API

First create a functional API.

const itFilter = p => function* (ix) {
  for (const x of ix)
    if (p(x))
      yield x;
};

const itMap = f => function* (ix) {
  for (const x of ix)
    yield f(x);
};

const itTake = n => function* (ix) {
  let m = n;
  
  for (const x of ix) {
    if (m-- === 0)
      break;

    yield x;
  }
};

const comp3 = f => g => h => x =>
  f(g(h(x)));    const xs = [1,2,3,4,5,6,7,8,9,10];

const stream = comp3(itTake(3))
  (itFilter(x => x % 2 === 0))
    (itMap(x => x * 3));

console.log(
  Array.from(stream(xs))
);

2. Box-Type

Next, define a Box type to allow method chaining for arbitrarily functional APIs.

function Box(x) {
  return new.target ? (this.x = x, this) : new Box(x)
}

Box.prototype.map = function map(f) {return new Box(f(this.x))};
Box.prototype.fold = function fold(f) {return f(this.x)};

3. Method Chaining

Finally, use the new Box type to chain methods.

const itFilter = p => function* (ix) {
  for (const x of ix)
    if (p(x))
      yield x;
};

const itMap = f => function* (ix) {
  for (const x of ix)
    yield f(x);
};

const itTake = n => function* (ix) {
  let m = n;
  
  for (const x of ix) {
    if (m-- === 0)
      break;
      
    yield x;
  }
};

const xs = [1,2,3,4,5,6,7,8,9,10];

function Box(x) {
  return new.target ? (this.x = x, this) : new Box(x)
}

Box.prototype.map = function map(f) {return new Box(f(this.x))};
Box.prototype.fold = function fold(f) {return f(this.x)};

const stream = Box(xs)
  .map(itMap(x => x * 3))
  .map(itFilter(x => x % 2 === 0))
  .map(itTake(3))
  .fold(x => x);
  
 console.log(
   Array.from(stream)
 );

Box gives you a fluent API for free.