Summary - Generators

JavaScript course · Advanced · Lesson 53/100

Build a function that generates an infinite number sequence using a generator.

Summary - Generators

In this group you learned the building blocks of generators. You start with function*, produce values with yield, and step through with .next(). You delegate to another generator with yield*, and write async generators with async function*.

Generators provide lazy evaluation: values are produced only when requested. This makes them ideal for infinite sequences and large data streams.

Exercise Write an infinite number sequence

Write a generator named naturalNumbers that produces natural numbers from 1, infinitely. Inside, use a while (true) loop, a let n = 1 counter, and a yield n; increment n each step. Create a generator object called nums and log the first three values via .next().value with console.log.

Show example solution

function* naturalNumbers() {
  let n = 1;
  while (true) {
    yield n;
    n++;
  }
}
const nums = naturalNumbers();
console.log(nums.next().value);
console.log(nums.next().value);
console.log(nums.next().value);

When to use generators

When should you use generators? They are ideal for infinite sequences: a generator with while (true) can produce endless values without consuming memory. Each value is computed only when .next() is called.

They are also useful for data streams and async iteration. When you want to process a large list in chunks without loading it all into memory, generators are a natural solution.

Exercise Write an ID maker generator

Write a generator named prefix that yields the string 'user'. Write a generator named idMaker that takes a start parameter, first delegates to prefix with yield* prefix(), then yields numbers from start in a while (true) loop. Create a generator called ids and log the first three values via .next().value with console.log.

Show example solution

function* prefix() {
  yield "user";
}
function* idMaker(start) {
  yield* prefix();
  let id = start;
  while (true) {
    yield id;
    id++;
  }
}
const ids = idMaker(1);
console.log(ids.next().value);
console.log(ids.next().value);
console.log(ids.next().value);

Test yourself

What happens if you use 'while (true)' inside a generator?
- It throws an error
- Memory overflows
- An infinite sequence of values is produced
- The generator never runs
Show answer

An infinite sequence of values is produced — Correct! You can write an infinite generator with while(true); values are produced only as .next() is called.
Review generator concepts

function*

defines a generator
yield

pauses and gives value
yield*

calls another generator
async function*

async generator

Summary - Generators