为什么异步数组映射返回承诺而不是值 [英] Why does async array map return promises, instead of values

问题描述

请参见下面的代码

var arr = await [1,2,3,4,5].map(async (index) => { 
    return await new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(index);
            console.log(index);
        }, 1000);
    });
});
console.log(arr); // <-- [Promise, Promise, Promise ....]
// i would expect it to return [1,2,3,4,5]

快速 可以接受的答案是正确的，可以说map对异步功能没有做任何特殊的事情.我不知道为什么我认为它可以识别异步fn并知道等待响应.

Quick edit: The accepted answer is correct, by saying that map doesnt do anything special to async functions. I dont know why i assumed it recognizes async fn and knows to await the response.

也许我期待这样的事情.

I was expecting something like this, perhaps.

Array.prototype.mapAsync = async function(callback) {
    arr = [];
    for (var i = 0; i < this.length; i++)
        arr.push(await callback(this[i], i, this));
    return arr;
};

var arr = await [1,2,3,4,5].mapAsync(async (index) => { 
    return await new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(index);
            console.log(index);
        }, 1000);
    });
});
// outputs 1, 2 ,3 ... with 1 second intervals, 
// arr is [1,2,3,4,5] after 5 seconds.

推荐答案

因为总是 函数会返回承诺； map没有异步性的概念，也没有对诺言的特殊处理.

Because an async function always returns a promise; and map has no concept of asynchronicity, and no special handling for promises.

但是您可以通过Promise.all随时等待结果:

But you can readily wait for the result with Promise.all:

try {
    const results = await Promise.all(arr);
    // Use `results`, which will be an array
} catch (e) {
    // Handle error
}

实时示例:

var arr = [1,2,3,4,5].map(async (index) => { 
    return await new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(index);
            console.log(index);
        }, 1000);
    });
});
(async() => {
    try {
        console.log(await Promise.all(arr));
        // Use `results`, which will be an array
    } catch (e) {
        // Handle error
    }
})();

.as-console-wrapper {
  max-height: 100% !important;
}

或使用Promise语法

or using Promise syntax

Promise.all(arr)
    .then(results => {
        // Use `results`, which will be an array
    })
    .catch(err => {
        // Handle error
    });

实时示例:

var arr = [1,2,3,4,5].map(async (index) => { 
    return await new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(index);
            console.log(index);
        }, 1000);
    });
});
Promise.all(arr)
    .then(results => {
        console.log(results);
    })
    .catch(err => {
        // Handle error
    });

.as-console-wrapper {
  max-height: 100% !important;
}

侧面说明:由于async函数总是返回promise，而函数中唯一要执行的await就是您创建的promise，因此无论如何在这里都不能使用async函数.只需返回您正在创建的承诺即可:

Side note: Since async functions always return promises, and the only thing you're awaiting in your function is a promise you create, it doesn't make sense to use an async function here anyway. Just return the promise you're creating:

var arr = [1,2,3,4,5].map((index) => { 
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(index);
            console.log(index);
        }, 1000);
    });
});

当然，如果您真的在其中做一些更有趣的事情，并且在各种事物上使用await(而不是仅仅在new Promise(...)上)，那就不一样了. :-)

Of course, if you're really doing something more interesting in there, with awaits on various things (rather than just on new Promise(...)), that's different. :-)

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