Because Node.js works asynchronously, things might not work as you think, if you are still stuck in the synchronous world..

To pass variables into callback function, you can use bind().

I will show you how.

The dummy function that takes 1 sec to callback

To begin, let’s say we have this dummy() function that will do some lengthy processing, and then callback. This is common. For example, a simple http request will use a callback function for the http response.

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// A dummy function that callback after 1 second (simulating some processing work)
function dummy(i, callback) {
  setTimeout(function() {
      // After 1 second, we callback with a result
      callback('dumb result')
    }, 1000);
}

Incorrect Code

Now, let’s have a code that loops 10 times.

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// Incorrect version
for (var i = 0; i < 10; i++) {
  dummy(i, function(response) {
      console.log("i = " + i + " , response = " + response);
  })
}

You might think the code above will print out i as 1-10.

But wrongly, it will print 10 every time!

This is because the callback from dummy() takes 1 second before calling back. So by the time we print out, i is already 10.

Correct Code

There is the correct way to pass variables such as i into the callback function.

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// Correct version
for (var i = 0; i < 10; i++) {
  dummy(i, function(response) {
      console.log("i = " + this.i + " , response = " + response);
  }.bind( {i: i} ))
}

As you can see, I have used bind() to pass the variables to function(response).

Then in the callback, I can access the variable with this.i.