AsyncGenerator.prototype.next()
The next()
method of AsyncGenerator
instances returns the next value in the sequence.
Syntax
js
next()
next(value)
Parameters
value
Optional-
An optional value used to modify the internal state of the generator. A value passed to the
next()
method will be received byyield
Return value
Examples
Using next()
The following example shows a simple generator and the object that the next
method returns:
js
// 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.
js
// 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();
Specifications
Specification |
---|
ECMAScript Language Specification # sec-asyncgenerator-prototype-next |
Browser compatibility
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