Question

192

How to structure javascript callback so that function scope is maintained properly

rated 0 times [ 193] [ 1] / answers: 1 / hits: 39692 / 14 Years ago, mon, may 24, 2010, 12:00:00

I'm using XMLHttpRequest, and I want to access a local variable in the success callback function.

Here is the code:

function getFileContents(filePath, callbackFn) {  

    var xhr = new XMLHttpRequest();

    xhr.onreadystatechange = function() {

        if (xhr.readyState == 4) {

            callbackFn(xhr.responseText);

        }

    }

    xhr.open(GET, chrome.extension.getURL(filePath), true);

    xhr.send();

}

And I want to call it like this:

var test = lol;



getFileContents(hello.js, function(data) {

    alert(test);

});

Here, test would be out of the scope of the callback function, since only the enclosing function's variables are accessible inside the callback function. What is the best way to pass test to the callback function so the alert(test); will display test correctly?

Edit:

Now, if I have the following code calling the function defined above:

for (var test in testers) {

    getFileContents(hello.js, function(data) {

        alert(test);

    });

}

The alert(test); code only prints the last value of test from the for loop. How do I make it so that it prints the value of test during the time at which the function getFileContents was called? (I would like to do this without changing getFileContents because it's a very general helper function and I don't want to make it specific by passing a specific variable like test to it.

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

kiarra

Add To Favorites

Follow

Total Points: 202

Total Questions: 111

Total Answers: 109

Location: Sudan

Member since Mon, May 31, 2021

3 Years ago

answered 14 Years ago wilson · Accepted Answer

With the code you have provided test will still be in scope inside the callback. xhr will not be, other than xhr.responseText being passed in as data.

Updated from comment:

Assuming your code looks something like this:

for (var test in testers)

  getFileContents(hello+test+.js, function(data) {

    alert(test);

  });

}

As this script runs, test will be assigned the values of the keys in testers - getFileContents is called each time, which starts a request in the background. As the request finishes, it calls the callback. test is going to contain the FINAL VALUE from the loop, as that loop has already finished executing.

There is a technique you can use called a closure that will fix this sort of problem. You can create a function that returns your callback function, creating a new scope you can hold onto your variables with:

for (var test in testers) {

  getFileContents(hello+test+.js, 

    (function(test) { // lets create a function who has a single argument test

      // inside this function test will refer to the functions argument

      return function(data) {

        // test still refers to the closure functions argument

        alert(test);

      };

    })(test) // immediately call the closure with the current value of test

  );

}

This will basically create a new scope (along with our new function) that will hold on to the value of test.

Another way of writing the same sort of thing:

for (var test in testers) {

  (function(test) { // lets create a function who has a single argument test

    // inside this function test will refer to the functions argument

    // not the var test from the loop above

    getFileContents(hello+test+.js, function(data) {

        // test still refers to the closure functions argument

        alert(test);

    });

  })(test); // immediately call the closure with the value of `test` from `testers`

}