循环原生承诺；

Question

我试图与本地异步循环ES6承诺这
样的 作品，但不正确。我想我在某个地方犯了一个大错误，我需要有人告诉我它在哪里以及如何正确完成

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

好的部分：它到了尽头。

不好的部分：它捕获了一些错误，我不知道为什么。

Answer 1

我见过的任何带有promise循环的辅助函数实际上使它变得比您可以使用递归开箱即用的方法差很多。

是的，.thenReturn但是更好一点：

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);
});

在jsfiddle中查看http://jsfiddle.net/fd1wc1ra/

这几乎等同于：

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

如果要进行嵌套循环，则完全相同：

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);
});