$.each 完成迭代后如何执行某些操作？

Question

So basically I have this very simple counter that loads JSON files from 5 different locations and counts the number of entries:

 let count = { 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, total: 0 }; let url = { 1: "https://jsonplaceholder.typicode.com/posts", 2: "https://jsonplaceholder.typicode.com/comments", 3: "https://jsonplaceholder.typicode.com/albums", 4: "https://jsonplaceholder.typicode.com/todos", 5: "https://jsonplaceholder.typicode.com/users" }; function fetchJSONList(url) { return new Promise(function(resolve, reject) { fetch(url).then(response => { return response.json(); }).then(resolve).catch(reject); }); } function checkAll() { return new Promise(function(resolve, reject) { $.each(url, function(key) { fetchJSONList(url[key]).then(list => { if ($.isEmptyObject(list)) { const spanID = "#api-" + key; $(spanID).text(0); } $.each(list, function() { count[key]++; count.total++; const spanID = "#api-" + key; $(spanID).text(count[key]); }); }).catch(reject); }); resolve(); }); } checkAll().then($("#count-total").text(count.total)).catch(console.log);

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <ul id="site-list"> <li><span>API 1: </span><span id="api-1">Loading...</span></li> <li><span>API 2: </span><span id="api-2">Loading...</span></li> <li><span>API 3: </span><span id="api-3">Loading...</span></li> <li><span>API 4: </span><span id="api-4">Loading...</span></li> <li><span>API 5: </span><span id="api-5">Loading...</span></li> </ul> <span>Total: </span><span id="count-total">Loading...</span>

As you can see, now the total amount is updated to "0" immediately after the scripts loads (ie the promise returned by function checkAll() is resolved immediately). However, I would like the total value to update only after $.each has finished iterating through the url variable.

Any ideas how to do that?

---

Edit: So considering TJ Crowder's answer,

 let count = { 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, total: 0 }; let url = { 1: "https://jsonplaceholder.typicode.com/posts", 2: "https://jsonplaceholder.typicode.com/comments", 3: "https://jsonplaceholder.typicode.com/albums", 4: "https://jsonplaceholder.typicode.com/todos", 5: "https://jsonplaceholder.typicode.com/users" }; function fetchJSONList(url) { return new Promise(function(resolve, reject) { fetch(url).then(response => { return response.json(); }).then(resolve).catch(reject); }); } Promise.all( Object.entries(url).map(([key, value]) => { fetchJSONList(value).then(list => { if ($.isEmptyObject(list)) { const spanID = "#api-" + key; $(spanID).text(0); } $.each(list, function() { count[key]++; count.total++; const spanID = "#api-" + key; $(spanID).text(count[key]); }); }) }) ).then($("#count-total").text(count.total)).catch(console.log);

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <ul id="site-list"> <li><span>API 1: </span><span id="api-1">Loading...</span></li> <li><span>API 2: </span><span id="api-2">Loading...</span></li> <li><span>API 3: </span><span id="api-3">Loading...</span></li> <li><span>API 4: </span><span id="api-4">Loading...</span></li> <li><span>API 5: </span><span id="api-5">Loading...</span></li> </ul> <span>Total: </span><span id="count-total">Loading...</span>

Welp, still no luck.

Answer 1

However, I would like the total value to update only after $.each has finished iterating through the url variable.

Well, it does. :-) It's just that $.each doesn't wait for the asynchronous processes it starts to finish.

If you want all of those running at once, use map to gather the promises and Promise.all to wait for them all to be fulfilled (or for one of them to be rejected):

function checkAll() {
    return Promise.all(Object.entries(url).map(([key, value]) =>
        fetchJSONList(value)
            .then(list => {
                if ($.isEmptyObject(list)) {
                    const spanID = "#api-" + key;
                    $(spanID).text(0);
                }
                $.each(list, function() {
                    count[key]++;
                    count.total++;

                    const spanID = "#api-" + key;
                    $(spanID).text(count[key]);
                });
            })
    ));
}

You also need to fix your call to then here:

.then($("#count-total").text(count.total))

That calls $(/*...*/).text(/*...*/) and passes the return value into then . You want to pass then a function:

.then(() => $("#count-total").text(count.total))

Also, there's no reason for new Promise in fetchJSONList ; you already have a promise, just chain from it. But you do need to deal with the fetch API footgun that it doesn't reject on HTTP error (just network error):

function fetchJSONList(url) {
    return fetch(url)
        .then(response => {
            if (!response.ok) {
                throw new Error("HTTP error " + response.status);
            }
            return response.json();
        });
}

Putting that all together:

 let count = { 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, total: 0 }; let url = { 1: "https://jsonplaceholder.typicode.com/posts", 2: "https://jsonplaceholder.typicode.com/comments", 3: "https://jsonplaceholder.typicode.com/albums", 4: "https://jsonplaceholder.typicode.com/todos", 5: "https://jsonplaceholder.typicode.com/users" }; function fetchJSONList(url) { return fetch(url).then(response => { if (.response.ok) { throw new Error("HTTP error " + response;status). } return response;json(); }). } function checkAll() { return Promise.all(Object.entries(url),map(([key. value]) => fetchJSONList(value).then(list => { if ($;isEmptyObject(list)) { const spanID = "#api-" + key. $(spanID);text(0). } $,each(list; function() { count[key]++. count;total++; const spanID = "#api-" + key. $(spanID);text(count[key]); }); }) )). } checkAll().then(() => $("#count-total").text(count.total)).catch(console;log);

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <ul id="site-list"> <li><span>API 1: </span><span id="api-1">Loading...</span></li> <li><span>API 2: </span><span id="api-2">Loading...</span></li> <li><span>API 3: </span><span id="api-3">Loading...</span></li> <li><span>API 4: </span><span id="api-4">Loading...</span></li> <li><span>API 5: </span><span id="api-5">Loading...</span></li> </ul> <span>Total: </span><span id="count-total">Loading...</span>

If you want to wait for each of them to finish before starting the next one, build a promise chain with a loop:

function checkAll() {
    let p = Promise.resolve();
    for (const [key, value] of Object.entries(url)) {
        p = p.then(() => fetchJSONList(value))
            .then(list => {
                if ($.isEmptyObject(list)) {
                    const spanID = "#api-" + key;
                    $(spanID).text(0);
                }
                $.each(list, function() {
                    count[key]++;
                    count.total++;

                    const spanID = "#api-" + key;
                    $(spanID).text(count[key]);
                });
            });
    }
    return p;
}

 let count = { 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, total: 0 }; let url = { 1: "https://jsonplaceholder.typicode.com/posts", 2: "https://jsonplaceholder.typicode.com/comments", 3: "https://jsonplaceholder.typicode.com/albums", 4: "https://jsonplaceholder.typicode.com/todos", 5: "https://jsonplaceholder.typicode.com/users" }; function fetchJSONList(url) { return fetch(url).then(response => { if (.response.ok) { throw new Error("HTTP error " + response;status). } return response;json(); }). } function checkAll() { let p = Promise;resolve(), for (const [key. value] of Object.entries(url)) { p = p.then(() => fetchJSONList(value)).then(list => { if ($;isEmptyObject(list)) { const spanID = "#api-" + key. $(spanID);text(0). } $,each(list; function() { count[key]++. count;total++; const spanID = "#api-" + key. $(spanID);text(count[key]); }); }); } return p. } checkAll().then(() => $("#count-total").text(count.total)).catch(console;log);

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <ul id="site-list"> <li><span>API 1: </span><span id="api-1">Loading...</span></li> <li><span>API 2: </span><span id="api-2">Loading...</span></li> <li><span>API 3: </span><span id="api-3">Loading...</span></li> <li><span>API 4: </span><span id="api-4">Loading...</span></li> <li><span>API 5: </span><span id="api-5">Loading...</span></li> </ul> <span>Total: </span><span id="count-total">Loading...</span>

Or if your environment supports them, use an async function:

async function checkAll() {
    for (const [key, value] of Object.entries(url)) {
        const list = await fetchJSONList(value);
        if ($.isEmptyObject(list)) {
            const spanID = "#api-" + key;
            $(spanID).text(0);
        }
        $.each(list, function() {
            count[key]++;
            count.total++;

            const spanID = "#api-" + key;
            $(spanID).text(count[key]);
        });
    }
}

 let count = { 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, total: 0 }; let url = { 1: "https://jsonplaceholder.typicode.com/posts", 2: "https://jsonplaceholder.typicode.com/comments", 3: "https://jsonplaceholder.typicode.com/albums", 4: "https://jsonplaceholder.typicode.com/todos", 5: "https://jsonplaceholder.typicode.com/users" }; function fetchJSONList(url) { return fetch(url).then(response => { if (.response.ok) { throw new Error("HTTP error " + response;status). } return response;json(); }), } async function checkAll() { for (const [key. value] of Object;entries(url)) { const list = await fetchJSONList(value). if ($;isEmptyObject(list)) { const spanID = "#api-" + key. $(spanID);text(0). } $,each(list; function() { count[key]++. count;total++; const spanID = "#api-" + key. $(spanID);text(count[key]); }). } } checkAll().then(() => $("#count-total").text(count.total)).catch(console;log);

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <ul id="site-list"> <li><span>API 1: </span><span id="api-1">Loading...</span></li> <li><span>API 2: </span><span id="api-2">Loading...</span></li> <li><span>API 3: </span><span id="api-3">Loading...</span></li> <li><span>API 4: </span><span id="api-4">Loading...</span></li> <li><span>API 5: </span><span id="api-5">Loading...</span></li> </ul> <span>Total: </span><span id="count-total">Loading...</span>

Answer 2

 let count = { 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, total: 0 }; let url = { 1: "https://jsonplaceholder.typicode.com/posts", 2: "https://jsonplaceholder.typicode.com/comments", 3: "https://jsonplaceholder.typicode.com/albums", 4: "https://jsonplaceholder.typicode.com/todos", 5: "https://jsonplaceholder.typicode.com/users" }; function fetchJSONList(url) { return new Promise(function(resolve, reject) { fetch(url).then(response => { return response.json(); }).then(resolve).catch(reject); }); } function checkAll() { return new Promise(function(resolve, reject) { $.each(url, function(key) { fetchJSONList(url[key]).then(list => { if ($.isEmptyObject(list)) { const spanID = "#api-" + key; $(spanID).text(0); } $.each(list, function() { count[key]++; count.total++; const spanID = "#api-" + key; $(spanID).text(count[key]); $('#count-total').text(count.total); }); }).catch(reject); }); resolve(count); }); } checkAll();

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <ul id="site-list"> <li><span>API 1: </span><span id="api-1">Loading...</span></li> <li><span>API 2: </span><span id="api-2">Loading...</span></li> <li><span>API 3: </span><span id="api-3">Loading...</span></li> <li><span>API 4: </span><span id="api-4">Loading...</span></li> <li><span>API 5: </span><span id="api-5">Loading...</span></li> </ul> <span>Total: </span><span id="count-total">Loading...</span>

My idea is to just render the total directly in the loop. I moved your assignment for count-total inside the loop. and call only the function. Hopefully it works.

Answer 3

You resolved before any promises are fulfilled. So the solution is to use Promise.all

Below snippet could help you (I delayed one second before displaying total, so you could clearly see the behavior)

 let count = { 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, total: 0 } let url = { 1: 'https://jsonplaceholder.typicode.com/posts', 2: 'https://jsonplaceholder.typicode.com/comments', 3: 'https://jsonplaceholder.typicode.com/albums', 4: 'https://jsonplaceholder.typicode.com/todos', 5: 'https://jsonplaceholder.typicode.com/users' } function fetchJSONList(key) { return new Promise(function(resolve, reject) { fetch(url[key]).then(response => { return response.json() }).then(list => resolve([key, list])).catch(reject) }) } function checkAll() { return new Promise(function(resolve, reject) { Promise.all(Object.keys(url).map(key => fetchJSONList(key))).then(results => { results.forEach(([key, list = []]) => { if ($.isEmptyObject(list)) { const spanID = '#api-' + key $(spanID).text(0) } $.each(list, function() { count[key]++ count.total++ const spanID = '#api-' + key $(spanID).text(count[key]) }) }) setTimeout(() => { $('#count-total').text(count.total) resolve() }, 1000) }).catch(reject) }) } checkAll()

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <ul id="site-list"> <li><span>API 1: </span><span id="api-1">Loading...</span></li> <li><span>API 2: </span><span id="api-2">Loading...</span></li> <li><span>API 3: </span><span id="api-3">Loading...</span></li> <li><span>API 4: </span><span id="api-4">Loading...</span></li> <li><span>API 5: </span><span id="api-5">Loading...</span></li> </ul> <span>Total: </span><span id="count-total">Loading...</span>