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JavaScript Function Generator - GeeksforGeeks

Last Updated : 11 Jul, 2025

A generator function is a special type of function that can pause its execution at any point and resume later. They are defined using the function* syntax and use the yield keyword to pause execution and return a value.

Syntax

function* generatorFunction() {
    // Code that can yield multiple values
}

function* generate() {
    yield 'Hello';
    yield 'World';
    return 'Done';
}

const generator = generate();


//Driver Code Starts
console.log(generator.next());
console.log(generator.next()); 
console.log(generator.next());
console.log(generator.next()); 
//Driver Code Ends

{ value: 'Hello', done: false }
{ value: 'World', done: false }
{ value: 'Done', done: true }
{ value: undefined, done: true }

• The next() method is used to resume execution and retrieve the next value.
• The done property indicates whether the generator has finished executing.

Generators work by implementing the iterator protocol. When a generator function is called, it does not execute its body immediately. Instead, it returns a special iterator object called a generator object. This object can be used to control the execution of the generator.

• Execution Control: The next() method resumes execution of the generator function until the next yield is encountered. It returns an object with two properties:
  • value: The yielded value.
  • done: A boolean indicating if the generator has completed execution.
• State Preservation: Each call to next() resumes the function from where it was last paused, retaining the function’s state.
• Termination: When the generator completes execution, the done property of the returned object becomes true.

function* example() {
    console.log("Step 1");
    yield 1;
    console.log("Step 2");
    yield 2;
    console.log("Step 3");
    return 3;
}

const gen = example();


//Driver Code Starts
console.log(gen.next());
console.log(gen.next());
console.log(gen.next());
//Driver Code Ends

Step 1
{ value: 1, done: false }
Step 2
{ value: 2, done: false }
Step 3
{ value: 3, done: true }

• Pause and Resume: Generators allow pausing function execution using yield and resuming it with the next() method.
• Iterable Interface: They return an iterator that conforms to the iterable protocol.
• Asynchronous Support: Ideal for working with asynchronous tasks using for-await-of loops.

Generators simplify the creation of custom iterators, making it easy to generate sequences of values.

function* fibonacci(limit) {
    let [prev, current] = [0, 1];
    while (limit--) {
        yield current;
        [prev, current] = [current, prev + current];
    }
}
const fib = fibonacci(5);
console.log([...fib]); 

In this example

• The function fibonacci is a generator function defined with function*.
• The generator starts with two variables prev and current, initialized to 0 and 1 (the first two Fibonacci numbers).
• The while (limit--) loop runs until the limit is exhausted, yielding the current Fibonacci number each time.
• The values of prev and current are updated after each yield to produce the next Fibonacci number.

Generators, in combination with libraries like co or with async/await syntax, help manage asynchronous flows.

function* asyncTask() {
    console.log('Task 1');
    yield new Promise(resolve =>
        setTimeout(() =>
            resolve('Task 2'), 1000));
    console.log('Task 3');
}
const task = asyncTask();
task.next().value.then(console.log);
task.next(); 

Output:

Task 1
Task 3
Task 2

In this example

• The generator asyncTask first logs 'Task 1'.
• It then yields a Promise that resolves with 'Task 2' after 1 second.
• After the Promise resolves, 'Task 3' is logged.

Generators can create infinite sequences that only compute values on demand.

function* infiniteSeq() {
    let i = 0;
    while (true) {
        yield i++;
    }
}
const sequence = infiniteSeq();

console.log(sequence.next().value); 
//Driver Code Starts
console.log(sequence.next().value); 
console.log(sequence.next().value); 
//Driver Code Ends

In this example

• The infiniteSeq generator creates an infinite sequence starting from 0.
• Each time next() is called, it yields the current value of i and then increments it (i++).
• The loop is infinite because of while (true), so it keeps generating numbers indefinitely.

• Lazy Evaluation: Values are computed only when needed, improving performance for large or infinite sequences.
• Readable Asynchronous Code: Generators can make asynchronous workflows look synchronous, simplifying complex code.
• Modularity: Encapsulate logic for producing sequences or iterating over data within a generator.
• Customizable Iterators: Generators allow creating iterators without implementing the entire iterable protocol manually.

• Complexity: Understanding and debugging generator-based code can be challenging for beginners.
• Not Fully Asynchronous: While useful, generators alone are not a replacement for promises or async/await in handling asynchronous programming.

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