顺序读取和5种解决方法

在技​​术面试中，除检查理论知识外，习惯上设置任务以评估应聘者的实践知识水平，其编写代码的能力以及逻辑和算法思维的能力。通常，此列表包括算法问题。每个人都已经习惯了它们，在准备时，首先，他们要看着他们。 列表很长，但是最常见的主要内容是这样的：

• 阶乘
• 斐波那契数
• 数组元素的唯一性
• 检查文本中的括号
• (mergeSort, insertionSort, bubbleSort, quickSort)
• ( / / )

, 70 JavaScript, , , , ( , ).

, :

, , .

:

, :

:
fetch(url1) => fetch(url2, resultsUrl1) => fetch(url3, resultsUrl2)

-
compose(res2 => fetch(url3, res2), res1 => fetch(url2, res1), () => fetch(url1))

?

, ( ):

• async/await

, , ( reduce, ).

, ; , - ; ; .

, . . , , , .

, reduce , Array. :

• reduce

, fetch , :

function fakeFetch (url, params='-') {
    //           
    console.log(`fakeFetch to: ${url} with params: ${params}`);
    return new Promise(resolve => {
        setTimeout(() => resolve(`${url} is DONE`), 1000);
    })
};

( ):

const urls = ['url1', 'url2', 'url3'];

( 1), . then().then().then() await; await; await; .

, callback. :

fetchSeries(result => console.log(`result: ${result}`))

, . , , .

, , . , , , .

function generatorWay(callback) {
    function* generateSequence() {
        let results;
        for (let i = 0; i < urls.length; i++) {
            results = yield fakeFetch(urls[i], results);
        }
        return results;
    }
    function execute(generator, yieldValue) {
        let next = generator.next(yieldValue);
        if (!next.done) {  
            return next.value
                .then(result => execute(generator, result));
        } else {
            callback(next.value);
        }
    }
    execute(generateSequence())
}

:

• generateSequence yield' , .
• execute(generator), next, — , , , next.
• execute , callback.

, while:

async function asyncGeneratorWay(callback) {
    async function* generateSequence() {
        let results;
        for (let i = 0; i < urls.length; i++) {
            results = yield await fakeFetch(urls[i], results);
        }
        return results;
    }
    let generator = generateSequence();
    let result;
    while (!result || !result.done) {
        result = await generator.next(result && result.value);
    }
    callback(result.value);
}

( ).

for await of , 2.

Async/await

. , async/await. , async, .

async function asyncAwaitWay(callback) {
    const series = async () => {
        let results;
        for (let i = 0; i < urls.length; i++) {  
            results = await fakeFetch(urls[i], results);
        }
        return results;
    }
    const result = await series();
    callback(result);
}

fakeFetch await;

Recursion

, reduce ( ), recursion . . , :

function recursionWay(callback) {  
    const recursion = (arr = [], promise = Promise.resolve()) => {
        if (!arr.length) { 
            return promise;
        }
        const [url, ...restUrls] = arr;
        return promise
            .then(res => recursion(restUrls, fakeFetch(url, res)));
    }
    recursion(urls)
        .then(result => callback(result));
}

shift , . .

Promise.resolve(), -, , , .

Reduce

, , . , , .

function reduceWay(callback) {
    urls
        .reduce((accum, item) => {
            return accum
                .then(res => fakeFetch(item, res))
        }, Promise.resolve())
        .then(result => callback(result));
}

:

• fakeFetch then;
• , Promise.resolve(), -, , (Promise) , . :

function reduceWay(callback) {
    urls
        .reduce((accum, item) => {
            if (!accum) {
                return fakeFetch(item);
            }
            return accum
                .then(res => fakeFetch(item, res));
        })
        .then(result => callback(result));
}

2 .

. , :

"", , reduce. , ( ). , , :

const reduceWay = callback => urls.reduce(
    (acc, item) => acc.then(res => fakeFetch(item, res)),  
    Promise.resolve())
    .then(result => callback(result));  
}

, , , .

, — . async/await — , . reduce / .

, , … .

发电机

异步发电机

数组：迭代减少

递归

异步/等待

承诺

承诺链

UPD在注释之后，我对async / await方法进行了一些调整。确实，可以使用以下方法来缩短该方法并提供更大的可见性for of：

async function asyncAwaitWay(callback) {
    let results;
    for (const url of urls) {  
        results = await fakeFetch(url, results);
        }
    callback(result);
}