async 和 await 关键字让我们可以用一种更简洁的方式写出基于 Promise 的异步行为，而无需刻意地链式调用 promise。
场景
Promise B需要接受 Promise A的返回值作为下一步计算的参数
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| function promiseA() {
return new Promise((resolve, reject) => {
resolve(1);
})
}
function promiseB(value) {
return new Promise((resolve, reject) => {
resolve(`value: ${value}`)
})
}
|
在没有async/await时，需要嵌套调用`Promise.then()`
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| promiseA().then(res => {
promiseB(res).then(res2 => {
console.log(res2)
})
})
|
使用`async/await`:
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| const fun = async () => {
const res = await promiseA();
const res2 = await promiseB(res);
console.log(res2)
}
fun()
|
对比起来，async/await 更像是在同步处理异步问题，更符合人的阅读直觉。
函数生成器模拟实现
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| function promise1() {
return new Promise((resolve) => {
setTimeout(() => {
resolve("1");
}, 1000);
});
}
function promise2(value) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve("value:" + value);
}, 1000);
});
}
function promise3(value) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve("value:" + value);
}, 1000);
});
}
function* readFile() {
const value = yield promise1();
const value2 = yield promise2(value);
const result = yield promise3(value2);
return result;
}
function asyncGen(fn) {
const g = fn();
return new Promise((resolve, reject) => {
function next(param) {
const { value, done } = g.next(param);
if (!done) {
Promise.resolve(value).then(
(res) => {
next(res);
},
(err) => {
reject(err);
}
);
} else {
resolve(param);
}
}
next();
});
}
asyncGen(readFile).then(
(res) => console.log(res),
(err) => console.log(err)
);
|
