Chalarangelo/30-seconds-of-code

---
title: Create a JavaScript generator function based on a condition
shortTitle: Generate while condition is met
type: tip
language: javascript
tags: [function,generator]
cover: yellow-sofa
excerpt: Learn how to create a generator function that keeps producing new values as long as the given condition is met.
listed: true
dateModified: 2024-01-22
---

Almost every generator function needs a **termination condition**. Instead of writing the same code over and over again, we can easily create a generator function that takes a **predicate function** as an argument and keeps producing new values as long as the condition is met.

Given a condition, we can **initialize the current value** using a provided `seed` value. Then, using a `while` loop we can iterate as long as the `predicate` function called with the current `val` returns `true`.

Inside the loop, we can use `yield` to return the current `val` and optionally receive a **new seed value**, `nextSeed`. Finally, we can use the `next` function to **calculate the next value** from the current `val` and the `nextSeed`.

```js
const generateWhile = function* (seed, predicate, next) {
  let val = seed;
  let nextSeed = null;
  while (predicate(val)) {
    nextSeed = yield val;
    val = next(val, nextSeed);
  }
  return val;
};

[...generateWhile(1, v => v <= 5, v => ++v)]; // [1, 2, 3, 4, 5]
```

> [!TIP]
>
> You can switch this code to work **until a condition is met** by simply changing the `while` loop condition to `!predicate(val)`.