The next() method of AsyncGenerator instances returns the next value in the sequence.
Syntax
next()
next(value)
Parameters
value- : An optional value used to modify the internal state of the generator. A value passed to the
next()method will be received byyield
- : An optional value used to modify the internal state of the generator. A value passed to the
Return value
A Promise which when resolved returns an Object with two properties:
done- : A boolean value:
trueif the generator is past the end of its control flow. In this casevaluespecifies the return value of the generator (which may be undefined).falseif the generator is able to produce more values.
- : A boolean value:
value- : Any JavaScript value yielded or returned by the generator.
Examples
Using next()
The following example shows a simple generator and the object that the next method returns:
// An async task. Pretend it's doing something more useful
// in practice.
function delayedValue(time, value) {
return new Promise((resolve, reject) => {
setTimeout(() => resolve(value), time);
});
}
async function* createAsyncGenerator() {
yield delayedValue(500, 1);
yield delayedValue(500, 2);
yield delayedValue(500, 3);
}
const asyncGen = createAsyncGenerator();
asyncGen.next().then((res) => console.log(res)); // { value: 1, done: false }
asyncGen.next().then((res) => console.log(res)); // { value: 2, done: false }
asyncGen.next().then((res) => console.log(res)); // { value: 3, done: false }
asyncGen.next().then((res) => console.log(res)); // { value: undefined, done: true }
Sending values to the generator
In this example, next is called with a value.
Note: The first call does not log anything, because the generator was not yielding anything initially.
// An async task. Pretend it's doing something more useful
// in practice.
function sleep(time) {
return new Promise((resolve, reject) => {
setTimeout(resolve, time);
});
}
async function* createAsyncGenerator() {
while (true) {
await sleep(500);
const value = yield;
console.log(value);
}
}
async function main() {
const asyncGen = createAsyncGenerator();
// No log at this step: the first value sent through `next` is lost
console.log(await asyncGen.next(1)); // { value: undefined, done: false }
// Logs 2: the value sent through `next`
console.log(await asyncGen.next(2)); // { value: undefined, done: false }
}
main();