CODEWITHSUNDEEP
javascriptsettimeout
Jatin Ganhotra
Jatin Ganhotra

Reputation: 7025

How does setTimeout works on Node.JS, please explain this code snippet

I was just getting ready to get my hands rough with Nodejs, but while experimenting and playing a bit on the first example, i found some behaviour which I am finding hard to understand. 

var a = function () {
    console.log('print 3');
    return 5000;
    };

setTimeout( function (){
   console.log('print 2');
   }, a()
);

console.log('print 1');

The output to the above code is:

print 3
print 1
print 2

AND a stupid doubt is, while the above code works, this one doesn't. 

setTimeout( function (){
   console.log('print 2');
   }, (function () {console.log('print 3'); return 5000;} )
);
console.log('print 1');

Please explain the above behaviour.

Upvotes: 1

Views: 3215

Answers (3)

Hogan
Hogan

Reputation: 70523

You need to call the 2nd parameter -- setTimeout expects a number

setTimeout( function (){
   console.log('print 2');
   }, (function () {console.log('print 3'); return 5000;} )()
);
console.log('print 1');

Upvotes: 1

Gazler
Gazler

Reputation: 84150

Formatting the code better should help you understand what is happening.

var a = function () {
  console.log('print 3');
  return 5000;
};

setTimeout( function (){
  console.log('print 2'); # Executed after 5000 ms.
  },
a());                    # Executed first and returns 5000

console.log('print 1');  #Executed second

The following snippet does not work as you never execute the 2nd part of the function.

setTimeout( function (){
  console.log('print 2');
  }, (function () {            # Never executed.
    console.log('print 3');
    return 5000;
  } )
);

console.log('print 1');

The following will work:

setTimeout( function (){
  console.log('print 2');
  }, (function () {            
    console.log('print 3');
    return 5000;
  } )();                      #The brackets () will execute the function.
);

console.log('print 1');

As an aside

The example has nothing to do with node.js and will produce the exact same results in any browser that responds to console.

Upvotes: 3

Quentin
Quentin

Reputation: 943515

setTimeout takes two arguments (at least, it does the way you are using it).

The first argument is a function to call, and the second argument is the time to delay before calling it.

In your first example, you pass a anonymous function and a Number (the return value of a()).

In your second example, you pass two anonymous functions (because you never call the second function (there is no ())).

setTimeout( 
    function (){
        console.log('print 2');
    },
    (function () {console.log('print 3'); return 5000;})()
);
console.log('print 1');

Upvotes: 1

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