Why does code below fs.readFile callback function run before the callback?

Question

fs.readFile('input.txt', function(err, data) {
  if (err) {
    return console.error(err);
  }
  console.log(data.toString());
});

console.log("Program Ended");

input.txt contains the string "hello".

The above code prints:

Program Ended
hello

Why does it print "program ended" before "hello"?
Is it not line by line execution?

Welcome to the world of asynchronous programming :) – demux Jan 09 '16 at 14:07 — demux, Jan 09 '16 at 14:07

score 2 · Answer 1 · answered Jan 09 '16 at 14:06

2

Code inside your callback function (the one which prints the file content) does not block execution. It is executed asynchronously after the file is read.

answered Jan 09 '16 at 14:06

madox2

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Best online resource for NODE? – Shah Rukh K Jan 09 '16 at 14:14
You can find more in this question: http://stackoverflow.com/questions/16336367/what-is-the-difference-between-synchronous-and-asynchronous-programming-in-node – madox2 Jan 09 '16 at 14:23

score 0 · Answer 2 · answered Jan 30 '21 at 19:20

fs.readFile is asynchronous, meaning that, as @madox2 said, it will not block execution. If you want to log Program Ended after fs.readFile, you can use fs.readFileSync. fs.readFileSync is synchronous, meaning that console.log("Program Ended") will be executed after.

Why does code below fs.readFile callback function run before the callback?

2 Answers2