JavaScript generator function that zips arbitrary number iterables…

/**
 * Zips values from multiple iterator/generator objects
 * @param {Iterator<any>[]} iterables
 * @yields {any[]}
 */
function* zipIterables(...iterables) {
  while (true) {
    const stats = [];
    const results = [];

    for (const iterable of iterables) {
      const result = iterable.next();
      stats.push(result.done);
      results.push(result.value);
    }

    if (stats.every((stat) => stat === true)) {
      break;
    }

    yield results;
  }
}

Few generator and iterator example functions…

/**
 * Converts string to character-by-character generator
 */
function* generatorFromString(str) {
  for (let character of [...str]) {
    yield character;
  }
}


/**
 * Converts array to element-by-element generator
 */
function* generatorFromArray(array) {
  for (let value of array) {
    yield value;
  }
}


/**
 * Iterator class that counts by specified amount
 */
class Counter {
  constructor({start = 0, step = 1, stop = Infinity}) {
    this.value = start;
    this.done = false;
    this.step = (step > 0 || step < 0) ? step : 1;

    this.stop = stop;
    if (isNaN(stop)) {
      this.stop = step > 0 ? Infinity : -Infinity;
    }
  }

  next() {
    if (this.stop === this.value) {
      this.done = true;
    }

    if (this.done) {
      this.value = undefined;
      return this;
    }

    this.value += this.step;
    return this;
  }

  [Symbol.iterator]() { return this; }
}

Example of initializing each function and class…

const counter = new Counter({stop: 18});

const iterate_string = generatorFromString('Spam flavored ham');

const iterate_array = generatorFromArray([
  'I',
  'II',
  'III',
  'IV',
  'V',
  'VI',
  'VII',
  'VIII',
  'IX',
  'X',
]);

Example usage and output…

const zipper = zipIterables(counter, iterate_string, iterate_array);


for (let values of zipper) {
  console.log({values});
}
//> { values: [ 1, 'S', 'I' ] }
//> { values: [ 2, 'p', 'II' ] }
//> { values: [ 3, 'a', 'III' ] }
//> { values: [ 4, 'm', 'IV' ] }
//> { values: [ 5, ' ', 'V' ] }
//> { values: [ 6, 'f', 'VI' ] }
//> { values: [ 7, 'l', 'VII' ] }
//> { values: [ 8, 'a', 'VIII' ] }
//> { values: [ 9, 'v', 'IX' ] }
//> { values: [ 10, 'o', 'X' ] }
//> { values: [ 11, 'r', undefined ] }
//> { values: [ 12, 'e', undefined ] }
//> { values: [ 13, 'd', undefined ] }
//> { values: [ 14, ' ', undefined ] }
//> { values: [ 15, 'h', undefined ] }
//> { values: [ 16, 'a', undefined ] }
//> { values: [ 17, 'm', undefined ] }
//> { values: [ 18, undefined, undefined ] }

Warning zipper may get stuck if any iterables fail to provide { done: true } result status

Contact and/or Contribute

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