Note:
这个答案只涵盖了串联await
和Promise.all
之间的时序差异.一定要读到@mikep's comprehensive answer that also covers the more important differences in error handling.
为了回答这个问题,我将使用一些示例方法:
res(ms)
是一个以毫秒为整数的函数,并返回在该整数毫秒之后解析的promise .
rej(ms)
是一个以毫秒为整数的函数,它返回一个promise ,但在那许多毫秒之后拒绝.
拨打res
会启动计时器.使用Promise.all
等待少量延迟将在所有延迟结束后解决,但请记住它们是同时执行的:
Example #1
const data = await Promise.all([res(3000), res(2000), res(1000)])
// ^^^^^^^^^ ^^^^^^^^^ ^^^^^^^^^
// delay 1 delay 2 delay 3
//
// ms ------1---------2---------3
// =============================O delay 1
// ===================O delay 2
// =========O delay 3
//
// =============================O Promise.all
async function example() {
const start = Date.now()
let i = 0
function res(n) {
const id = ++i
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve()
console.log(`res #${id} called after ${n} milliseconds`, Date.now() - start)
}, n)
})
}
const data = await Promise.all([res(3000), res(2000), res(1000)])
console.log(`Promise.all finished`, Date.now() - start)
}
example()
这意味着Promise.all
将在3秒后与来自内部promise 的数据进行解析.
但是,Promise.all
has a "fail fast" behavior:
Example #2
const data = await Promise.all([res(3000), res(2000), rej(1000)])
// ^^^^^^^^^ ^^^^^^^^^ ^^^^^^^^^
// delay 1 delay 2 delay 3
//
// ms ------1---------2---------3
// =============================O delay 1
// ===================O delay 2
// =========X delay 3
//
// =========X Promise.all
async function example() {
const start = Date.now()
let i = 0
function res(n) {
const id = ++i
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve()
console.log(`res #${id} called after ${n} milliseconds`, Date.now() - start)
}, n)
})
}
function rej(n) {
const id = ++i
return new Promise((resolve, reject) => {
setTimeout(() => {
reject()
console.log(`rej #${id} called after ${n} milliseconds`, Date.now() - start)
}, n)
})
}
try {
const data = await Promise.all([res(3000), res(2000), rej(1000)])
} catch (error) {
console.log(`Promise.all finished`, Date.now() - start)
}
}
example()
如果改为使用async-await
,则必须等待每个promise 按顺序解析,这可能效率较低:
Example #3
const delay1 = res(3000)
const delay2 = res(2000)
const delay3 = rej(1000)
const data1 = await delay1
const data2 = await delay2
const data3 = await delay3
// ms ------1---------2---------3
// =============================O delay 1
// ===================O delay 2
// =========X delay 3
//
// =============================X await
async function example() {
const start = Date.now()
let i = 0
function res(n) {
const id = ++i
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve()
console.log(`res #${id} called after ${n} milliseconds`, Date.now() - start)
}, n)
})
}
function rej(n) {
const id = ++i
return new Promise((resolve, reject) => {
setTimeout(() => {
reject()
console.log(`rej #${id} called after ${n} milliseconds`, Date.now() - start)
}, n)
})
}
try {
const delay1 = res(3000)
const delay2 = res(2000)
const delay3 = rej(1000)
const data1 = await delay1
const data2 = await delay2
const data3 = await delay3
} catch (error) {
console.log(`await finished`, Date.now() - start)
}
}
example()