Synchronous for loop

[gupta....@gmail.com]

Hi
 I am new to node js. I am facing an issue.
I have 3 ids. I am iterating in for loop. For each value I am calling web service. The web service is find by id by passing each id. What I noticed is that it always takes the last id.
for eg: id are 10, 20, 30
When I call any service by passing in for loop it always takes 30.
Any help is appreciated.

Regards

[DaneiL]

Can u post an example?

Create gist or put some example code, so we can inspect what could be wrong in your code?

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[Mark Volkmann]

You want something like this assuming whatever you use to call the web service, fetch in my example, returns a promise.

const ids = [23, 17, 45];

const promises = ids.map(i’d => fetch(urlPrefix + id));

const results - Promise.all(promises);

You’ll have to correct the URLs being passed to fetch, but that should get you started.

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Object Computing, Inc.

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[gupta....@gmail.com]

Here is my code. Its a simple one
 for(var a=0; a<dResponse.length;a++ ){
        var dId = dResponse[a].dId;
        console.log('dId-->' +dResponse);
          var dUrl = url?id=' + dId;
          console.log('Url:'+dUrl);
    }
Every time it returns the same url.

[Mark Volkmann]

There is nothing async in your loop, so it doesn’t make sense that you could get the same output from each iteration unless every object in dResponse is the same. Are you saying that you get the value of dResponse from an async call? You didn’t show that part.

Also, you are making the code to iterate through dResponse too complicated. Try something like this:

for (const res of dResponse) {

  const {dId} = res;

  console.log(‘did =‘, dId);

  const dUrl = `${url}?id=${dId}`;
  console.log(‘dUrl =‘, dUrl);

}

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R. Mark Volkmann

Object Computing, Inc.

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[Zlatko]

On Wednesday, October 25, 2017 at 7:38:37 AM UTC+2, gupta....@gmail.com wrote:

Here is my code. Its a simple one
 for(var a=0; a<dResponse.length;a++ ){
        var dId = dResponse[a].dId;
        console.log('dId-->' +dResponse);
          var dUrl = url?id=' + dId;
          console.log('Url:'+dUrl);
    }
Every time it returns the same url.

You didn't show the whole code - what's in dResponse array? But, your title says "Synchronous for loop", depending on your node version, here's a simple example:

const request = require('request');

function callRemoteApi(url) {

  return new Promise((resolve, reject) => {

    console.log('Fetching', url);

    request(url, (err, res) => {

      if (err) {

        return reject(err);

      }

      console.log('Got', url);

      const result = JSON.parse(res.body).url;

      resolve(result);

    });

  });

}

async function runTest() {

  const urls = [

    'https://httpbin.org/delay/5',

    'https://httpbin.org/delay/1',

    'https://httpbin.org/delay/3',

  ];

  console.log('Urls ready.');

  let promises = [];

  for (let url of urls) {

    promises.push(callRemoteApi(url));

  };

  console.log('Promises ready.');

  await Promise.all(promises.map(async promise => console.log('Result of promise:' + await promise)))

    .then(() => console.log('All done.'));

}

runTest();

Now, if you run this code, it'll show you how things are working.

So the for loop example is indicative of potential issues with sync for loops. It takes an url, waits for it to return, and then goes on to the next.

Now, the other example works much better: we execute all promises immediately, in parallel. And as they come back, we get the results logged immediately.

Not sure how applicable this is to your situation, but if you post more code, we might help out.

 

[Arun Antony]

Use each method in underscore library.

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[gupta....@gmail.com]

Thank you for the response. Below is my code:
for(var i=0;i<response.length;i++){
        var messageTimeStamp = response[i].messageTimeStamp;
        console.log('messageTimeStamp-->'+messageTimeStamp);
        var body = {deviceId:req.body.deviceId};
        console.log('response1-->',response[i]);
        Custody.find(body, function(error, custodyResponse) {
        if(custodyResponse.length == 1){
          console.log('custodyResponse.length-->'+custodyResponse.length);
          console.log('response2-->',response[i]);

          }

        });

here I am getting value for response1 but not for response2

Regards

On Tuesday, October 24, 2017 at 4:57:58 PM UTC-4, gupta....@gmail.com wrote:

[Mark Volkmann]

You are using the loop index i inside a function that is invoke asynchronously. Try changing this:

console.log('response2-->',response[i]);

to this:

console.log('response2-->',response[0]);

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Object Computing, Inc.

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