WTF are Javascript Generators?

- 4 min read

Hello, JavaScript adventurers! Today, we’re going to explore an often-underutilized feature of JavaScript that can bring a whole new level of flexibility to your code - JavaScript Generators. If you’re already comfortable with JavaScript’s quirks and features but haven’t delved into generators yet, this is your perfect starting point. Let’s unravel this mystery together!

JavaScript Generators: What Are They?

To put it simply, generators are special functions that can pause and resume their execution. Imagine being able to put a bookmark inside your function, and you’ll get the idea!

						function* generatorFunction() {
    console.log('This will run first.');
    yield 'Hello, ';

    console.log('I will print after pause');
    yield 'World!';

In this snippet, generatorFunction is a generator, as indicated by the function* syntax and the yield keyword.


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Seeing Generators in Action

Now, let’s witness this pause-and-resume feature in action.

						const generator = generatorFunction();

console.log(; // { value: 'Hello, ', done: false }
console.log(; // { value: 'World!', done: false }
console.log(; // { value: undefined, done: true }

Each next() call makes the generator function run until it hits the next yield. Then it pauses and returns the yielded value. When there are no more yield statements, next() returns { value: undefined, done: true }, indicating the end of the generator’s task.

Unleashing the Full Potential of Generators

You might be wondering, “Interesting, but what’s the practical use?” Let’s delve into how generators can simplify your life as a developer.

  • Asynchronous Operations

Tired of dealing with callback hell or a maze of Promises? Generators are your savior! They can manage asynchronous operations, making your code appear as if it’s running synchronously, thus enhancing readability.

						function* fetchUser(userId) {
  const response = yield fetch(`${userId}`);
  const user = yield response.json();

  return user;

Here, fetchUser is a generator that fetches a user from an API. It pauses at each yield until the asynchronous operation completes, then resumes with the result.

  • State Management

Generators are excellent for managing state. If you’ve ever juggled multiple variables to track state in your functions, you’ll appreciate the simplicity generators bring.

						function* idGenerator() {
  let id = 0;

  while (true) {
    yield ++id;

In this example, idGenerator is a generator that generates a new ID each time it’s called. The ID is stored in the generator’s state, so it’s automatically saved and updated each time the generator yields.

Generators in Real-World: Effect-TS Library

For a real-world application of generators, consider the Effect-TS library. In a nutshell, Effect is a Typescript library designed to help you to easily create complex, synchronous and asynchronous programs. It leverages the concept of Effect pattern to create a powerful ecosystem of composable tool that allow you to write your logic in an effectfull way.

This library uses generators to create and manage effects, as a convenient syntax, similar to async/await, using this approach you can write Effectfull logic in a way you are already use to:

						import { Effect } from "effect"

const increment = (x: number) => x + 1

const divide = (a: number, b: number): Effect.Effect<never, Error, number> =>
  b === 0
    ? Error("Cannot divide by zero"))
    : Effect.succeed(a / b)

// $ExpectType Effect<never, never, number>
const task1 = Effect.promise(() => Promise.resolve(10))
// $ExpectType Effect<never, never, number>
const task2 = Effect.promise(() => Promise.resolve(2))

// $ExpectType Effect<never, Error, string>
export const program = Effect.gen(function* (_) {
  const a = yield* _(task1)
  const b = yield* _(task2)
  const n1 = yield* _(divide(a, b))
  const n2 = increment(n1)
  return `Result is: ${n2}`

Effect.runPromise(program).then(console.log) // Output: "Result is: 6"

In this example, program is a generator function that describes an effect. The yield* syntax delegates to another generator/effect

Then after all the logic was described, the program is run using Effect.runPromise

Wrapping Up

JavaScript generators offer a powerful way to simplify your code, making it more readable and manageable. They’re especially handy for handling asynchronous operations and managing state. And with libraries like Effect-TS leveraging generators, the possibilities are endless.

So, next time you’re grappling with callbacks or state variables, consider using a generator. You might find it’s the tool you’ve been seeking. Happy coding!

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