Chalarangelo/30-seconds-of-code

---
title: How can I create a JavaScript generator function that repeats one or more values indefinitely?
short: Generate values repeatedly
type: question
language: javascript
tags: [function,generator]
cover: secret-tree
excerpt: Learn how to create a generator function that repeats a given value or an array of values indefinitely.
listed: true
dateModified: 2024-02-06
---

Generator functions are extremely versatile, especially when it comes to creating **sequences of values**. You can use them to easily create a generator that repeats a given value or an array of values indefinitely.

## Repeating a single value

To create a generator that **repeats a single value** indefinitely, you can use a non-terminating `while` loop, that will `yield` a value every time `Generator.prototype.next()` is called. You can also use the return value of the `yield` statement to **update the returned value** if the passed value is not `undefined`.

```js
const repeatGenerator = function* (val) {
  let v = val;
  while (true) {
    let newV = yield v;
    if (newV !== undefined) v = newV;
  }
};

const repeater = repeatGenerator(5);
repeater.next(); // { value: 5, done: false }
repeater.next(); // { value: 5, done: false }
repeater.next(4); // { value: 4, done: false }
repeater.next(); // { value: 4, done: false }
```

## Repeating an array of values

To create a generator that **repeats an array of values** indefinitely, you can use the same approach as before for the non-terminating `while` loop and `yield` a value every time `Generator.prototype.next()` is called. In order to **cycle through the array**, you can use the modulo operator (`%`) with `Array.prototype.length` to get the next value's index and increment the counter after each `yield` statement.

```js
const cycleGenerator = function* (arr) {
  let i = 0;
  while (true) {
    yield arr[i % arr.length];
    i++;
  }
};

const binaryCycle = cycleGenerator([0, 1]);
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
```