Javascript Array#map() 中的异步/等待
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Async/Await inside Array#map()
提问by MyTitle
I'm getting compile time error with this code:
我收到此代码的编译时错误：
const someFunction = async (myArray) => {
return myArray.map(myValue => {
return {
id: "my_id",
myValue: await service.getByValue(myValue);
}
});
};
Error message is:
错误信息是：
await is a reserved word
await 是一个保留字
Why can't I use it like this?
为什么我不能像这样使用它？
I also tried another way, but it gives me same error:
我也尝试了另一种方式，但它给了我同样的错误：
const someFunction = async (myArray) => {
return myArray.map(myValue => {
const myNewValue = await service.getByValue(myValue);
return {
id: "my_id",
myValue: myNewValue
}
});
};
回答by lonesomeday
You can't do this as you imagine, because you can't use awaitif it is not directly inside an asyncfunction.
你不能像你想象的那样做，因为await如果它不是直接在async函数内部，你就不能使用。
The sensible thing to do here would be to make the function passed to mapasynchronous. This means that mapwould return an array of promises. We can then use Promise.allto get the result when all the promises return. As Promise.allitself returns a promise, the outer function does not need to be async.
在这里做的明智的事情是将函数传递给map异步。这意味着map将返回一组承诺。然后我们可以使用Promise.all来获取所有承诺返回时的结果。由于Promise.all本身返回一个承诺，外部函数不需要是async。
const someFunction = (myArray) => {
const promises = myArray.map(async (myValue) => {
return {
id: "my_id",
myValue: await service.getByValue(myValue)
}
});
return Promise.all(promises);
}
回答by helb
That's because the function in mapisn't async, so you can't have awaitin it's return statement. It compiles with this modification:
那是因为 in 函数map不是async，所以你不能在它的 return 语句中使用await。它通过以下修改进行编译：
const someFunction = async (myArray) => {
return myArray.map(async (myValue) => { // <-- note the `async` on this line
return {
id: "my_id",
myValue: await service.getByValue(myValue)
}
});
};
So… it's not possible to give recommendation without seeing the rest of your app, but depending on what are you trying to do, either make the innerfunction asynchronous or try to come up with some different architecture for this block.
所以……在没有看到应用程序的其余部分的情况下不可能给出推荐，但是根据您要做什么，要么使内部函数异步，要么尝试为这个块提出一些不同的架构。
Update: we might get top-level await one day: https://github.com/MylesBorins/proposal-top-level-await
更新：有一天我们可能会获得顶级等待：https: //github.com/MylesBorins/proposal-top-level-await
回答by MattCochrane
If you want to run map with an asynchronous mapping function you can use the following code:
如果要使用异步映射函数运行 map，可以使用以下代码：
const resultArray = await Promise.all(inputArray.map(async (i) => someAsyncFunction(i)));
How it works:
这个怎么运作：
inputArray.map(async ...)returns an array of promises - one for each value ininputArray.- Putting
Promise.all()around the array of promises converts it into a single promise. - The single promise from
Promise.all()returns an array of values - the individual promises each resolve to one value. - We put
awaitin front ofPromise.all()so that we wait for the combined promise to resolve and store the array of resolved sub-promises into the variableresultArray.
inputArray.map(async ...)返回一组 Promise - 中的每个值一个inputArray。- 把
Promise.all()承诺将其转换的阵列周围成一个单一的承诺。 - 来自单个承诺
Promise.all()返回一组值 - 每个承诺都解析为一个值。 - 我们把
await放在前面，Promise.all()以便我们等待组合承诺解析并将已解析的子承诺数组存储到变量中resultArray。
In the end we get one output value in resultArrayfor each item in inputArray, mapped through the function someAsyncFunction. We have to wait for all async functions to resolve before the result is available.
最后，我们resultArray为 中的每个项目获得一个输出值，并inputArray通过函数映射someAsyncFunction。在结果可用之前，我们必须等待所有异步函数解析。
