JavaScript Generators

JavaScript generators are a powerful feature of the language, enabling developers to write better asynchronous code. They provide a way to pause function execution and resume it at will, making them particularly useful for managing flow in applications. In this guide, we delve into the world of JavaScript generators, exploring their syntax, use cases, and advanced techniques to enhance your programming skills.

Understanding Generator Functions

The Basics of Generator Functions

A generator function is declared similar to a regular function but uses the function* syntax. The star (*) indicates that the function is a generator. Here’s a simple example of a generator function:

function* simpleGenerator() {

When called, a generator function does not execute its code. Instead, it returns a special type of iterator, known as a generator. You can control this iterator to execute segments of the generator function’s code:

function* simpleGenerator() { yield 'Hello'; yield 'World'; } const generator = simpleGenerator(); console.log(; // { value: 'Hello', done: false } console.log(; // { value: 'World', done: false } console.log(; // { done: true } no value here

Controlling the Flow

Generators are particularly valuable for controlling execution flow. You can pause the execution of the function at each yield and resume it from the outside. They remember their state, i.e., the variables and their values, between re-entries.

Advanced Generator Patterns

Using yield*

In complex scenarios, you might want to delegate part of the generator’s control to another generator. This is where yield* comes into play. It allows a generator to yield the values from another generator, iterable, or array.

function* numberGen() { yield* [1, 2, 3]; } const numbers = numberGen(); console.log(; // 1 console.log(; // 2 console.log(; // 3

Sending Values into Generators

Generators not only can yield values but also receive them via the next() method. This makes them interactive and allows the external input to affect their behavior.

function* interactiveGen() { const name = yield 'What is your name?'; yield `Hello, ${name}!`; } const greet = interactiveGen(); console.log(; // 'What is your name?' console.log('Alice').value); // 'Hello, Alice!'

Practical Applications of JavaScript Generators

Managing Asynchronous Operations

Generators can simplify the handling of asynchronous operations by avoiding the “callback hell” scenario. When combined with Promises, they allow for asynchronous code that is both easy to write and read.

function* fetchUser() { const uri = ''; const response = yield fetch(uri); const profile = yield response.json(); yield profile; } const userGenerator = fetchUser(); => { => { console.log(profile); // JSON object with user data }); });

Implementing Infinite Loops

Generators are excellent for implementing infinite loops that are both efficient and easy to control, which can be useful in scenarios such as game development or simulations.

function* infiniteSequence() { let index = 0; while (true) { yield index++; } } const sequence = infiniteSequence(); console.log(; // 0 console.log(; // 1


JavaScript generators offer a robust set of tools for managing execution flow, handling asynchronous code, and controlling complex logic. By understanding and utilizing generators, developers can write cleaner, more efficient JavaScript code. Whether you're managing API calls, handling user interactions, or simply need a clean way to manage intervals, generators provide a powerful solution to enhance your JavaScript projects.

Practice Your Knowledge

What is the function of the '*' character before the function keyword in JavaScript generators?

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