blocks|key|3999435|text|您可以使用递归或Promises来代替for..loop。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3999436|递归：：|3999437|let+insertToDB+=+function+(records,+cb)+{
++if+(!records.length)+return+cb();
++let+record+=+records.shift();//assuming+records+is+an+array
++//+Insert+record+into+DB1
++insertToDB(records,+cb);
};

let+loop+=+function+(count,+offset,+cb)+{
+if+(!count)+return+cb();
+fetch(
++++`https://api.example.io/api/v1/assets/?offset=${offset}`
+)
+insertToDB(recordsFromFetch,+function+()+{
++++offset+%2B=+20;
++++--count;
++++loop(count,+offset,+cb)
+})
};


loop(50,+0,+function+()+{
++console.log("All+Done");
})|code-block|syntax|javascript|3999438|Promise:：我还没有运行它，所以可能有一些语法错误，但是您应该明白了|3999439|let+insertToDB+=+function+(record)+{
++++return+new+Promise(function+(resolve,+reject)+{
++++//+Insert+record+into+DB+then+call+resolve
++++resolve();
++++})
};

let+fetchPhase+=+function+(offset)+{
++++fetch(
++++`https://api.example.io/api/v1/assets/?offset=${offset}`
+++)
++++let+dbAll+=+[];
+++for+(let+j+=+0;+j+<+20;+j%2B%2B)+{
++++//+assuming+you+get+records+from+fetch+,+pass+record+to+be+added+to+db+in+the+parameter
++++dbAll.push(insertToDB(records[j]))
}
+return+Promise.all(dbAll)
};

let+all+=+[];

let+offset+=+0;

for+(let+i+=+0;+i+<+50;+i%2B%2B)+{
++++all.push(fetchPhase(i));
}

Promise.all(all)
.then(function+()+{
++++console.log("ALL+DONe");
})|3999440|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Instead of for..loop, you could use recursion or Promises.

Recursion::

<pre><code>let insertToDB = function (records, cb) {
 if (!records.length) return cb();
 let record = records.shift();//assuming records is an array
 // Insert record into DB1
 insertToDB(records, cb);
};

let loop = function (count, offset, cb) {
 if (!count) return cb();
 fetch(
 `https://api.example.io/api/v1/assets/?offset=${offset}`
 )
 insertToDB(recordsFromFetch, function () {
 offset += 20;
 --count;
 loop(count, offset, cb)
 })
};


loop(50, 0, function () {
 console.log("All Done");
})
</code></pre>

Promise:: I have not run it, so might be some syntactical error, but you get the idea

<pre><code>let insertToDB = function (record) {
 return new Promise(function (resolve, reject) {
 // Insert record into DB then call resolve
 resolve();
 })
};

let fetchPhase = function (offset) {
 fetch(
 `https://api.example.io/api/v1/assets/?offset=${offset}`
 )
 let dbAll = [];
 for (let j = 0; j &lt; 20; j++) {
 // assuming you get records from fetch , pass record to be added to db in the parameter
 dbAll.push(insertToDB(records[j]))
}
 return Promise.all(dbAll)
};

let all = [];

let offset = 0;

for (let i = 0; i &lt; 50; i++) {
 all.push(fetchPhase(i));
}

Promise.all(all)
.then(function () {
 console.log("ALL DONe");
})
</code></pre>

blocks|key|3999484|text|为什么不使用Promise.all|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3999485|const+promises+=+[];

for+(let+i+=+0;+i+<+10;+i%2B%2B)+{
++++promises.push(
++++fetch(`https://api.example.io/api/v1/assets/?offset=${i}`)
++);
}

Promise.all(promises).then(+_results+=>+{
++++//+All+fetch+calls+have+completed+at+this+point.+_results+will+be+in+array+of+your+fetch+results.
++++console.log(results);
++++//+Do+db+insert+here

});|code-block|syntax|javascript|3999486|你可以做的一件事就是做一个承诺的function。在该函数中，让它执行fetch，然后执行db+insert+all+in+one函数(使用.then)。如果您这样做，那么单个Promise.all调用就可以处理所有事情。如果不这样做，您将不得不再次循环promises并将这些值插入到数据库中。它可能看起来像这样：|3999487|const+promises+=+[];

for+(let+i+=+0;+i+<+10;+i%2B%2B)+{
++++promises.push(fetchAndInsert(i));
}

Promise.all(promises).then(+_results+=>+{
++++console.log(results);

});


function+fetchAndInsert(offset)+{
++++return+new+Promise(+(resolve,+reject)+=>+{
++++fetch(`https://api.example.io/api/v1/assets/?offset=${i}`).then+(_result+=>+{
++++++++//+db.insert().then(+_result+=>+{
++++++++++++//resolve();
++++++//})
++++})
++})
}|3999488|entityMap^0|6|B|0|0|G|8|1Y|5|2G|B|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|P|$]]

Why not use <code>Promise.all</code>?

<pre><code>const promises = [];

for (let i = 0; i &lt; 10; i++) {
 promises.push(
 fetch(`https://api.example.io/api/v1/assets/?offset=${i}`)
 );
}

Promise.all(promises).then( _results =&gt; {
 // All fetch calls have completed at this point. _results will be in array of your fetch results.
 console.log(results);
 // Do db insert here

});
</code></pre>

One thing you could do is to make a <code>function</code> that is a promise. Within that function, have it execute the fetch and then execute the db insert all in one function (using <code>.then</code>). If you did it this way, the single <code>Promise.all</code> call would handle everything. If you don't, you'll have to loop through the promises again and insert those values into the db. It might look something like this:

<pre><code>const promises = [];

for (let i = 0; i &lt; 10; i++) {
 promises.push(fetchAndInsert(i));
}

Promise.all(promises).then( _results =&gt; {
 console.log(results);

});


function fetchAndInsert(offset) {
 return new Promise( (resolve, reject) =&gt; {
 fetch(`https://api.example.io/api/v1/assets/?offset=${i}`).then (_result =&gt; {
 // db.insert().then( _result =&gt; {
 //resolve();
 //})
 })
 })
}
</code></pre>

blocks|key|4002553|text|你可以使用async和await。这应该是可行的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4002554|async+function+fetchExample()+{
++for+(let+i+=+0;+i+<+50;+i%2B%2B)+{
++++let+fetchedData+=+await+fetch(
++++++`https://api.example.io/api/v1/assets/?offset=${offset}`
++++++);
++++for(data+of+fetchedData)+{
++++++//+Insert+into+DB
++++}
++offset%2B=20;
++}
}|code-block|syntax|javascript|4002555|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You could use async and await. This should work: 

<pre><code>async function fetchExample() {
 for (let i = 0; i &lt; 50; i++) {
 let fetchedData = await fetch(
 `https://api.example.io/api/v1/assets/?offset=${offset}`
 );
 for(data of fetchedData) {
 // Insert into DB
 }
 offset+=20;
 }
}
</code></pre>

blocks|key|3999592|text|我从我的一个nodeJS代码库中删除了以下代码，因为它使用异步代码来解决非常类似的问题：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3999593|//+Throttling+is+important+because+you+don't+want+to
//+overload+the+API
const+createThrottle+=+require('async-throttle');
const+throttle+=+createThrottle(2);

//+First+push+all+the+links+into+an+array+using
//+the+offset
const+links+=+[];
for+(let+offset+=+0;+offset+<+100;+offset+%2B=+20)+{
++links.push(`https://api.example.io/api/v1/assets/?offset=${offset}`);
}

//+Next+create+an+array+of+promises+by+`map`ing
//+over+the+links+using+`fetch`.
//+Notice+I've+used+throttle+here+to+slow+down+the+hits+on
//+the+API
const+promises+=+links.map(link+=>+throttle(async+=>+()+{
++const+res+=+await+fetch(link);
++return+await+res.json();
}));

//+Once+all+the+promises+are+complete,+iterate+over+their+datasets
Promise.all(promises).then(datasets+=>+{
++//+iterate+over+datasets
});|code-block|syntax|javascript|3999594|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I've nicked the following from one of my nodeJS repos as it employs async code for solving a very similar problem:

<pre><code>// Throttling is important because you don't want to
// overload the API
const createThrottle = require('async-throttle');
const throttle = createThrottle(2);

// First push all the links into an array using
// the offset
const links = [];
for (let offset = 0; offset &lt; 100; offset += 20) {
 links.push(`https://api.example.io/api/v1/assets/?offset=${offset}`);
}

// Next create an array of promises by `map`ing
// over the links using `fetch`.
// Notice I've used throttle here to slow down the hits on
// the API
const promises = links.map(link =&gt; throttle(async =&gt; () {
 const res = await fetch(link);
 return await res.json();
}));

// Once all the promises are complete, iterate over their datasets
Promise.all(promises).then(datasets =&gt; {
 // iterate over datasets
});
</code></pre>

blocks|key|3999598|text|Promise.all(Array(50).fill(null).map((v,i)+=>+{
++++const+url+=+`https://api.example.io/api/v1/assets/?offset=${i*20}`;

++++return+fetch(url).then(results+=>+{
++++++++for+(let+result+of+results)+{
++++++++++++//+insert+into+DB
++++++++}
++++);
}).then(()+=>+console.log("done"));|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3999599|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>Promise.all(Array(50).fill(null).map((v,i) =&gt; {
 const url = `https://api.example.io/api/v1/assets/?offset=${i*20}`;

 return fetch(url).then(results =&gt; {
 for (let result of results) {
 // insert into DB
 }
 );
}).then(() =&gt; console.log("done"));
</code></pre>

My scenario is as follows:
There is an API I would like to fetch from. The API returns a json that has an array named "assets". This array size will always be 20.
Now, I call the endpoint like so:

<pre><code> fetch(
 `https://api.example.io/api/v1/assets/?offset=${offset}`
 )
</code></pre>

where if offset is 0 it will return the array of the 0 - 20 assets, if the offset is 20 it will return 20 to 40 and so on.

I want to check 1000 items which means I would like to call this fetch 1000/20 = 50 times.
Whenever I call the fetch I would like loop over these 20 items and insert them into my DB.
The problem is that I can't do something like this:

<pre><code>let offset=0;

for(let i = 0; i &lt; 50; i++ {
 fetch(
 `https://api.example.io/api/v1/assets/?offset=${offset}`
 )
 for(let j = 0; j &lt; 20; j++){
 // Insert into DB
 }
 offset+=20;
}
</code></pre>

Due to JS asynchronous nature. Whenever I try to do this it will call the fetch with the value 0 for offset a couple of times, it won't wait until the nested for loop finishes and than call it for 20 and later 40 and so on...

What is the correct way to achive this behavior?

Nested for loop combined with fetching data from an API in Nodejs?

数据库

我的场景如下:有一个我想要获取的API。API返回一个json，该json有一个名为"assets“的数组。该数组大小始终为20。现在，我这样调用端点： fetch(     `https://api.example.io/api/v1/assets/?offset=${offset}` )其中，如果offset为0，...

问嵌套的for循环结合了Nodejs中的API获取数据？
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