Generator Functions
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function* name(): Generator<YieldType, ReturnType, NextType> {
yield value;
}Basic Generators
A generator function pauses execution at each yield and resumes only when next is called again — its return type Generator<TYield, TReturn, TNext> can describe all three of the values involved in that exchange.
Example: Basic Generators
function* countUp(): Generator<number, void, unknown> {
yield 1;
yield 2;
yield 3;
}
console.log([...countUp()]);Generator Return Values
A generator can return a final value once it completes normally (via a return statement inside the generator body), and the second type parameter of Generator describes exactly what type that return value is.
Example: Generator Return Values
function* gen(): Generator<number, string, unknown> {
yield 1;
yield 2;
return "done";
}
const it = gen();
console.log(it.next(), it.next(), it.next());Passing Values to Generators
The third Generator type parameter describes the type of values passed into the generator through next(value) — note the very first call to next starts the generator running, so any argument passed to that first call is ignored.
Example: Passing Values to Generators
function* echo(): Generator<string, void, string> {
const input = yield "first";
console.log("Received:", input);
}
const it = echo();
console.log(it.next());
it.next("hello");Generators for Data Processing
Generators are ideal for producing sequences lazily, meaning each value is only computed when it's actually requested, rather than eagerly computing an entire array up front.
Example: Generators for Data Processing
function* lazyRange(end: number): Generator<number> {
for (let i = 0; i < end; i++) yield i;
}
for (const n of lazyRange(3)) {
console.log(n);
}Generators and Iterable
Generator objects automatically implement the iterable protocol, so they can be consumed directly with for...of, spread into an array, or destructured — no extra wiring required.
Example: Generators and Iterable
function* fruits(): Generator<string> {
yield "apple";
yield "banana";
}
console.log([...fruits()]);
const [first] = fruits();
console.log(first);- Forgetting the
*infunction* countUp, soyieldis a syntax error. - Expecting the first
next(value)argument to be received, when it is discarded because the generator has not reached ayieldyet. - Iterating a generator twice and getting nothing on the second pass, since it is already exhausted.
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