Promise basics
- Promises start in a pending state, neither fulfilled or rejected.
- When the operation is completed, a promise will become fulfilled with a value.
- If the operation fails, a promise will get rejected with an error.
Creating promises
- The function passed to the
Promise
constructor will execute synchronously.
- Use
resolve()
or reject()
to create promises from values.
Promise.resolve(val)
will fulfill the promise with val
.
Promise.reject(err)
will reject the promise with err
.
- If you put a fulfilled promise into a fulfilled promise, they will collapse into one.
代码实现
// Resolving with a value, rejecting with an error
new Promise((resolve, reject) => {
performOperation((err, val) => {
if (err) reject(err);
else resolve(val);
});
});
// Resolving without value, no need for reject
const delay = (ms) => new Promise(resolve => setTimeout(resolve, ms));
Handling promises
-
Promise.prototype.then()
accepts two optional arguments (onFulfilled
, onRejected
).
-
Promise.prototype.then()
will call onFulfilled
once the promise is fulfilled.
-
Promise.prototype.then()
will call onRejected
if the promise is rejected.
-
Promise.prototype.then()
passes errors through if onRejected
in undefined.
-
Promise.prototype.catch()
accepts one argument (onRejected
).
-
Promise.prototype.catch()
behaves like Promise.prototype.then()
when onFulfilled
is omitted.
-
Promise.prototype.catch()
passes fulfilled values through.
-
Promise.prototype.finally()
accepts one argument (onFinally
).
-
Promise.prototype.finally()
calls onFinally
with no arguments once any outcome is available.
-
Promise.prototype.finally()
passes through input promise.
使用样例
promisedOperation()
.then(
val => value + 1, // Called once the promise is fulfilled
err => { // Called if the promise is rejected
if (err === someKnownErr) return defaultVal;
else throw err;
}
)
.catch(
err => console.log(err); // Called if the promise is rejected
)
.finally(
() => console.log('Done'); // Called once any outcome is available
);
- All three of the above methods will not be executed at least until the next tick, even for promises that already have an outcome.
Combining promises
Promise.all()
turns an array of promises into a promise of an array.
- If any promise is rejected, the error will pass through.
Promise.race()
passes through the first settled promise.
Promise
.all([ p1, p2, p3 ])
.then(([ v1, v2, v3 ]) => {
// Values always correspond to the order of promises,
// not the order they resolved in (i.e. v1 corresponds to p1)
});
Promise
.race([ p1, p2, p3 ])
.then(val => {
// val will take the value of the first resolved promise
});
async/await
- Calling an
async
function always results in a promise.
(async () => value)()
will resolve to value
.
(async () => throw err)()
will reject with an error.
await
waits for a promise to be fulfilled and returns its value.
await
can only be used in async
functions.
await
also accepts non-promise values.
await
always waits at least until the next tick before resolving, even when waiting already fulfilled promises or non-promise values.
async () => {
try {
let val = await promisedValue();
// Do stuff here
} catch (err) {
// Handle error
}
}
翻译自:https://www.30secondsofcode.org/js/s/async-cheatsheet