get variable value in callback function

Question

I have one callback function

function QueryKeyword(keyword, site, callback) {
  var querykeyword = keyword;
  var website = site;

  $.ajax({
    url: "http://www.test.com",
    jsonp: "jsonp",
    dataType: "jsonp",
    data: {
      Query: querykeyword
    },
    success: callback
  });
}

I am calling this function with in for loop like this :

for (i = 0; i < questionTerm.length; i++) {
  for (j = 0; j < site.length; j++) {
    var searchTerm = questionTerm[i] + ' ' + $('#search').val();

    QueryKeyword(searchTerm, site[j], function(reslt) {
      // I need to get j variable value here
      console.log(j);
    });

  }

}

Now I need to get "j" variable value in function see I console the j variable value but it does not get the j variable value.

Would you please let me know how I can fetch the value in this.

Thanks in advance

What exactly means for you "It doesnt get the j variable value". Is it undefined? Its a wrong value? Is it null? — Cristian S.
– Cristian S., Commented May 22, 2018 at 14:56
You should declare your variables properly. In this case, declaring them with let will fix the problem because j will then be block-scoped, and each callback function will close over its own, unique j. — user2437417
– user2437417, Commented May 22, 2018 at 14:56
Possible duplicate of JavaScript closure inside loops – simple practical example — Taplar
– Taplar, Commented May 22, 2018 at 15:01

Luke · Accepted Answer · 2018-05-22 15:14:37Z

4

The problem is, that at the moment of your callback, j was reassigned multiple times to something different.

There are a few options you could do.

call your callback with the params you need

function QueryKeyword(keyword, site, index, callback) {
  // ...
  $.ajax(
      success: function(result) {
          // call the callback with a second param (the index j)
          callback(result, index);  
      }
  )
}

QueryKeyword(searchTerm, site[j], j, function(reslt, param) {
   // param is j
   console.log(result, param);
});

save the var in a closure

(function() {
    var value = j;
    ...
})();

use forEach

questionTerm.forEach((term, i) => {
    site.forEach((s, j) => {
        // we are in a closure, 
        // j will be correct here.
        QueryKeyword(term, s, function(reslt) {
          // j is still correct here
          console.log(j);
        });
    })
});

if you use es6, you could use let keyword. Here is some good explanation, how it works when using for loops

for(let i = 0; i < 10; i++) {
 console.log(i);
 setTimeout(function() {
   console.log('The number is ' + i);
 },1000);
}

edited May 22, 2018 at 15:14

answered May 22, 2018 at 15:00

Luke

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3 Comments

Dhaval Over a year ago

When I use FOREACH then get "J" value but not true because when I call the function then make ajax call so each executes faster so return value like 0,3. But i need value like 0,1,2,3 like this.

Luke Over a year ago

so you need the exact order? do you also want all xhr calls to be in the right order? or why do you need console.log to be in that order?

Dhaval Over a year ago

Yes, I need index value of "site" variable in QueryKeyword() function becuase i need to perform some operation so is it possible to fetch "J" variable value(index of "site" variable)

dmgig · Accepted Answer · 2018-05-22 14:57:47Z

0

You have to pass it in separately:

definition

function QueryKeyword(keyword, site, index, callback)
{
   ...
}

execution

QueryKeyword(searchTerm, site[j], j, function(reslt) {
   // I need to get j variable value here
   console.log(j);
});

answered May 22, 2018 at 14:57

dmgig

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11 Comments

user2437417 Over a year ago

You don't "have to" do that though it's an option, but you're missing a vital step to make it work.

dmgig Over a year ago

What's the step? And why tell him to use let? How do you know he's using ES6?

user2437417 Over a year ago

I didn't tell him to use let. I only stated that using it would make it work. The missing step is passing the index into the callback when it gets invoked.

Dhaval Over a year ago

I got "J" value but not getting properly like 0,3 but i need a 0,1,2,3 like this sequence so i use async:false but it's not working with jsonp. If anyone have solution please help me. Thanks

Dhaval Over a year ago

Foreach loop is working for me. Work likes a charm. Thanks

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