Description
The asynchronous iterations proposal is now in stage 3, so it should be safe to support that in TypeScript now. Currently, a function may be a generator or an async function, but it cannot be both. The asynchronous iteration proposal removes this restriction. Here's an example from their readme:
async function* readLines(path) {
let file = await fileOpen(path);
try {
while (!file.EOF) {
yield await file.readLine();
}
} finally {
await file.close();
}
}Types
Generator functions declare several interfaces:
interface IteratorResult<T> {
done: boolean;
value: T;
}
interface Iterator<T> {
next(value?: any): IteratorResult<T>;
return?(value?: any): IteratorResult<T>;
throw?(e?: any): IteratorResult<T>;
}
interface Iterable<T> {
[Symbol.iterator](): Iterator<T>;
}
interface IterableIterator<T> extends Iterator<T> {
[Symbol.iterator](): IterableIterator<T>;
}Async generators return promises instead of their IteratorResult, and use a new symbol, Symbol.asyncIterator:
interface IteratorResult<T> {
done: boolean;
value: T;
}
interface AsyncIterator<T> {
next(value?: any): Promise<IteratorResult<T>>;
return?(value?: any): Promise<IteratorResult<T>>;
throw?(e?: any): Promise<IteratorResult<T>>;
}
interface AsyncIterable<T> {
[Symbol.asyncIterator](): AsyncIterator<T>;
}
interface AsyncIterableIterator<T> extends AsyncIterator<T> {
[Symbol.asyncIterator](): AsyncIterableIterator<T>;
}Emit
I guess that this would require another helper function to downlevel to ES2015, similar to the one used for async functions. Yield and await expression would both be replaced by yield expression, so they need to be decorated with some tag, like yield ['await', promise] for await promise.
for-await-of
The proposal also introduces a new loop. An example from their readme:
for await (const line of readLines(filePath)) {
console.log(line);
}This iterates over the [Symbol.asyncIterator]() object and awaits the value of .next(), similar to a for-of loop. This loop can only be used in async functions.