JavaScript Lazy Sequences

Written by Swapnil RajaPublished 24 Aug 2026

A generator can represent a sequence that has no end, because it only computes what is asked for.

An array has to exist in memory all at once; a generator does not.

Values are produced one at a time, only when requested.

Example

javascript

function* naturalNumbers() {
  let n = 1;
  while (true) {
    yield n;
    n++;
  }
}

const numbers = naturalNumbers();

console.log(numbers.next().value);
console.log(numbers.next().value);
console.log(numbers.next().value);

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Example

javascript

function* naturalNumbers() {
  let n = 1;
  while (true) {
    yield n;
    n++;
  }
}

function take(iterable, count) {
  const result = [];
  const iterator = iterable[Symbol.iterator]();
  for (let i = 0; i < count; i++) {
    result.push(iterator.next().value);
  }
  return result;
}

console.log(take(naturalNumbers(), 5).join(","));

Example

javascript

function* cycle(values) {
  let i = 0;
  while (true) {
    yield values[i % values.length];
    i++;
  }
}

function take(iterable, count) {
  const result = [];
  const iterator = iterable[Symbol.iterator]();
  for (let i = 0; i < count; i++) {
    result.push(iterator.next().value);
  }
  return result;
}

console.log(take(cycle(["a", "b", "c"]), 7).join(","));

Example

javascript

function* naturalNumbers() {
  let n = 1;
  while (true) {
    yield n;
    n++;
  }
}

let first = null;
for (const n of naturalNumbers()) {
  first = n;
  break;
}

console.log(first);

Example

javascript

function* naturalNumbers() {
  let n = 1;
  while (true) {
    yield n;
    n++;
  }
}

function* evensOnly(source) {
  for (const n of source) {
    if (n % 2 === 0) yield n;
  }
}

function take(iterable, count) {
  const result = [];
  const iterator = iterable[Symbol.iterator]();
  for (let i = 0; i < count; i++) {
    result.push(iterator.next().value);
  }
  return result;
}

console.log(take(evensOnly(naturalNumbers()), 4).join(","));

Example

javascript

function* naturalNumbers() {
  let n = 1;
  while (true) {
    yield n;
    n++;
  }
}

const iterator = naturalNumbers();

console.log(typeof iterator.next);
console.log(iterator.next().value);

Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript Lazy Sequences</title>
</head>
<body>

  <h1>Lazy Sequences</h1>

  <p id="out"></p>

  <script>
    function* naturalNumbers() {
      let n = 1;
      while (true) {
        yield n;
        n++;
      }
    }

    function take(iterable, count) {
      const result = [];
      const iterator = iterable[Symbol.iterator]();
      for (let i = 0; i < count; i++) {
        result.push(iterator.next().value);
      }
      return result;
    }

    document.getElementById("out").textContent =
      "First 10 numbers: " + take(naturalNumbers(), 10).join(", ");
  </script>

</body>
</html>

JavaScript Lazy Sequences

Example

Why This Is Safe

Syntax

A take Helper

Example

An Infinite Sequence of Anything

Example

Never Spread an Infinite Generator

Example

Filtering a Lazy Sequence

Example

Why Not Just Use a Huge Array

Example

Complete Example

Try It Yourself

Important Points

Conclusion