循环与本地承诺; [英] Loop with native promises;

问题描述

我正在尝试使用本机进行异步循环ES6承诺它 的作品，但不正确。我想我在某个地方犯了一个很大的错误，我需要有人告诉我它在哪里，以及它是如何正确地完成的。

I'm trying to make an asynchronous loop with native ES6 promises It kind of works, but incorrectly. I suppose I made a huge mistake somewhere and I need someone to tell me where it is and how it's done correctly

var i = 0;

//creates sample resolver
function payloadGenerator(){
    return function(resolve) {
        setTimeout(function(){
            i++;
            resolve();
        }, 300)
    }
}

// creates resolver that fulfills the promise if condition is false, otherwise rejects the promise.
// Used only for routing purpose
function controller(condition){
    return function(resolve, reject) {
        console.log('i =', i);
        condition ? reject('fin') : resolve();
    }
}

// creates resolver that ties payload and controller together
// When controller rejects its promise, main fulfills its thus exiting the loop
function main(){
    return function(resolve, reject) {
        return new Promise(payloadGenerator())
            .then(function(){
                return new Promise(controller(i>6))
            })
            .then(main(),function (err) {
                console.log(err);
                resolve(err)
            })
            .catch(function (err) {
                console.log(err , 'caught');
                resolve(err)
            })
    }
}


new Promise(main())
    .catch(function(err){
        console.log('caught', err);
    })
    .then(function(){
        console.log('exit');
        process.exit()
    });

现在输出：

/usr/local/bin/iojs test.js
i = 1
i = 2
i = 3
i = 4
i = 5
i = 6
i = 7
fin
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
error: [TypeError: undefined is not a function]
caught [TypeError: undefined is not a function]
exit

Process finished with exit code 0

很好的一部分：它结束了。

The good part: it reaches the end.

坏的部分：它捕获一些错误，我不知道为什么。

The bad part: it catches some errors and I don't know why.

推荐答案

任何帮助函数有希望循环我已经看到实际上使它比你可以做的开箱即用的递归更糟糕n。

Any helper function with promise looping I have seen actually made it much worse than what you can do out of the box with recursion.

这是一个更好的 .thenReturn 但是：

function readFile(index) {
    return new Promise(function(resolve) {
        setTimeout(function() {
            console.log("Read file number " + (index +1));
            resolve();
        }, 500);
    });
}

// The loop initialization
Promise.resolve(0).then(function loop(i) {
    // The loop check
    if (i < len) {              // The post iteration increment
        return readFile(i).thenReturn(i + 1).then(loop);
    }
}).then(function() {
    console.log("done");
}).catch(function(e) {
    console.log("error", e);
});

请参阅jsfiddle http://jsfiddle.net/fd1wc1ra/

See it in jsfiddle http://jsfiddle.net/fd1wc1ra/

这完全相当于：

try {
    for (var i = 0; i < len; ++i) {
        readFile(i);
    }
    console.log("done");
} catch (e) {
    console.log("error", e);
}

---

如果你想做嵌套循环完全一样：

---

If you wanted to do nested loops it is exactly the same:

http://jsfiddle.net/ fd1wc1ra / 1 /

function printItem(item) {
    return new Promise(function(resolve) {
        setTimeout(function() {
            console.log("Item " + item);
            resolve();
        }, 500);
    });
}

var mdArray = [[1,2], [3,4], [5,6]];
Promise.resolve(0).then(function loop(i) {
    if (i < mdArray.length) {
        var array = mdArray[i];
        return Promise.resolve(0).then(function innerLoop(j) {
            if (j < array.length) {
                var item = array[j];
                return printItem(item).thenReturn(j + 1).then(innerLoop);
            }
        }).thenReturn(i + 1).then(loop);
    }
}).then(function() {
    console.log("done");
}).catch(function(e) {
    console.log("error", e);
});

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