[FIXED] What is this syntax in NodeJS?

Issue

I am learning NodeJS, coming from other languages (C#, etc) and I find some of the syntax to be confusing.

For example this piece of code:

 for(var index in files) {
    console.log("-->"+index);
    var task = (function(file) { 
      return function() {
        fs.readFile(file, function(err, text) {
          if (err) throw err;
          countWordsInText(text);
          checkIfComplete();
        });
      }
    })(filesDir + '/' + files[index]);
    tasks.push(task); 
  }

What is this? var task= (function(file){return function(){......}})(filesDir+.....);

There is a function that is calling a function and suddenly some parameters outside?

I am guessing it is defining a list of functions but what is the rule for this syntax?

Solution

It's called IIFE (Immediately Invoked Function Expression). Basically, you define a function

function (){}

and immediately execute it

(function(){})();

What the code you've posted is doing is, executing a function and storing the returned value in task.

Answered By - Manish Gharat

Answer Checked By - Candace Johnson (PHPFixing Volunteer)