等待promise for循环 [英] Wait promise inside for loop

问题描述

let currentProduct;

for (let i = 0; i < products.length; i++) { 
    currentProduct = products[i];

    subscription.getAll(products[i]._id)
        .then((subs) => {
            update(subs, currentProduct);
        });
}

我使用的是bluebird，方法 getAll 和 update 返回承诺.我如何说等到两个承诺返回，然后更新currentProduct值"?我对JS很陌生...

I'm using bluebird, the methods getAll and update return promises. How can I say "Wait until the two promises return, then update the currentProduct value"? I'm quite new to JS...

推荐答案

如果您可以使用async/await:

// Make sure that this code is inside a function declared using
// the `async` keyword.
let currentProduct;

for (let i = 0; i < products.length; i++) { 
    currentProduct = products[i];

    // By using await, the code will halt here until
    // the promise resolves, then it will go to the
    // next iteration...
    await subscription.getAll(products[i]._id)
        .then((subs) => {
            // Make sure to return your promise here...
            return update(subs, currentProduct);
        });

    // You could also avoid the .then by using two awaits:
    /*
    const subs = await subscription.getAll(products[i]._id);
    await update(subs, currentProduct);
    */
}

或者，如果您只能使用简单的承诺，则可以遍历所有产品，并将每个承诺放在最后一个循环的.then中.这样，它只有在前一个问题解决后才会前进到下一个问题(即使它首先迭代了整个循环):

Or if you can only use plain promises, you can loop through all your products, and put each promise in the .then of the last loop. In that way, it will only advance to the next when the previous has resolved (even though it will have iterated the whole loop first):

let currentProduct;

let promiseChain = Promise.resolve();
for (let i = 0; i < products.length; i++) { 
    currentProduct = products[i];

    // Note that there is a scoping issue here, since
    // none of the .then code runs till the loop completes,
    // you need to pass the current value of `currentProduct`
    // into the chain manually, to avoid having its value
    // changed before the .then code accesses it.

    const makeNextPromise = (currentProduct) => () => {
        // Make sure to return your promise here.
        return subscription.getAll(products[i]._id)
            .then((subs) => {
                // Make sure to return your promise here.
                return update(subs, currentProduct);
            });
    }

    // Note that we pass the value of `currentProduct` into the
    // function to avoid it changing as the loop iterates.
    promiseChain = promiseChain.then(makeNextPromise(currentProduct))
}

在第二个代码段中，循环仅设置了整个链，但没有立即执行.then中的代码.您的getAll函数要等到每个先前的函数依次解决后才能运行(这就是您想要的).

In the second snippet, the loop just sets up the entire chain, but doesn't execute the code inside the .then immediately. Your getAll functions won't run until each prior one has resolved in turn (which is what you want).

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