A JavaScript Function can only return one value.
A JavaScript Generator can return multiple values, one by one.
A JavaScript Generator can yield a stream of data.
A JavaScript Generator can be paused and resumed.
Generator Functions
A Generator Function is defined using the function* syntax:
function* myGenerator() {}
When called, a generator function returns a Generator Object, not a direct value:
{value:1, done:false}
Generator Objects
A Generator Object is returned by the generator function.
It conforms to both the iterable and iterator protocols, meaning it can be iterated over using for...of loops.
Example
function* myStream() {
// return {value:1, done:false}
yield 1;
// return {value:2, done:false}
yield 2;
// return {value:3, done:true}
return 3;
}
// Create a Generator
let myGenerator = myStream();
// Iterate over the Generator
for (let value of myGenerator) { // code }
The yield Keyword
The yield keyword pauses execution and yield a value back to the caller.
The generator's state is preserved, and can be resumed from the point of the yield when next() is called.
Generators maintain their internal state between yield calls, allowing them to continue execution from where they left off.
Note: The code example above returns 1 and 2. Not 3! The for..of loop exits when done:true. To get all the values, return them with yield.
Example
function* myStream() {
// return {value:1, done:false}
yield 1;
// return {value:2, done:false}
yield 2;
// return {value:3, done:false}
yeald 3;
}
// Create a Generator
let myGenerator = myStream();
// Iterate over the Generator
for (let value of myGenerator) { // code }
Generator Object Methods
A Generator Object conforms to the iterable and iterator protocols.
It can be iterated over with for...of loops.
It also has methods:
| Method | Description |
|---|---|
| next() | Resumes the next execution of a generator function |
| return() | Finishes this execution and returns a given value |
| throw() | Throws an exeption in this execution |
The next() Method
When called on the Generator object it resumes execution of the generator function.
It returns an object with and value and a boolean, indicating if the generator has finished:
{ value: value, done: true|false}
Custom Iterators
Generators simplify the creation of custom iterators for complex data structures or sequences.
They can efficiently generate values on demand, making them suitable for potentially infinite data streams.
They can pause and resume execution, which can be useful for fine-grained control over program flow.
Before async/await, generators were used with Promises to manage asynchronous operations in a more sequential-looking manner.
Note: Generators bring the iteration concept directly into the core JavaScript language and provide a mechanism for customizing the behavior of for...of.