function with callback execute callback before function complete

Question

I'd write the following code to practice function with callback in javascript.

fs = require('fs');

function funcWithCallback(callback) {

  fs.readFile('YouBikeTP.txt', 'utf8', function (err,data) {
  if (err) {
    return console.log(err);
  }
  console.log(data.length);
  });
  callback();
}

funcWithCallback(function(){
  console.log("string in callback ")
})

The purpose of the code is try to control the sequence of methods execution. The string "string in callback" should be printed after the length of text file be printed, but when I ran this code the result will be:

>> "string in callback"
>> 91389 //the length of YouBikeTP.txt

which is not the result I expected. should be

 >> 91389 //the length of YouBikeTP.txt
 >> "string in callback"

could anyone tell me why will the callback function been called before funcWithCallback(callback) complete ? Did I misunderstand the meaning of callback function ?

Rahul Arora · Accepted Answer · 2017-06-20 05:44:39Z

4

Change your code to this:

Reason: the function you have defined as a callback in the readFile is an asynchronous callback. It doesn't execute right away, rather it executes when the file loading has completed. Therefore you need to call the main callback function after the console.log for async callback is done.

fs = require('fs');

function funcWithCallback(callback) {

   fs.readFile('YouBikeTP.txt', 'utf8', function (err,data) {
      if (err) {
          return console.log(err);
      }
      console.log(data.length);
      callback(); //calling the main callback after the async callback console logs
   });

}

funcWithCallback(function(){
    console.log("string in callback ")
})

answered Jun 20, 2017 at 5:44

Rahul Arora

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2 Comments

張軒銘 Over a year ago

thank you for replying , I have a further question use the same code above. If I add an IO-bound task (eq. save data to another file or a time consuming loop operation ) in the readFile callback at the previous line of callback() (the line that you commanded), will the callback() execute until the I/O-bound task finish ?

Rahul Arora Over a year ago

yes @張軒銘 whatever you are trying to do, if its synchronous, it will work fine i.e the callback will execute after that task as per my answer. But if the task you are doing is asynchronous, then you will have to place your main callback inside the callback for the big task that you are trying to accomplish. You can also use promises developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

Damiano · Accepted Answer · 2017-06-20 05:48:36Z

1

You are not calling callback () inside readFile's own callback, you are just calling it after calling readFile, that then calls its callback function (err,data) only after completing.

answered Jun 20, 2017 at 5:48

Damiano

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Comments

Naveen Kerati · Accepted Answer · 2017-06-29 11:28:58Z

1

you must call the callback() in the fs.readFile callback function.As the callback should be called after asyc function written with the result or error.

edited Jun 29, 2017 at 11:28

answered Jun 20, 2017 at 5:49

Naveen Kerati

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