NodeJs: Loops, Closure and Async Invocation

Closure

Closure is something that is very familiar to all C# developers who know lambda expressions/functions but they just may not know that it is called Closure in JavaScript.
Lets look at what is closure in JavaScript without further ado with an example:

function outer(){
	var inOuter = "outer";
	inner();
	
	function inner(){
		console.log(inOuter); // prints 'outer'
	}
}

In this example, the variable inOuter is defined in the function outer but it is used in the function inner. This is possible because, the a function in JavaScript can access all the variables in the scope where the function is defined and this is called Closure.

Loop and Closure

Lets consider the following example:

function outer(){
	for(var i=1; i<=5; i++){
		inner();
	}
	
	function inner(){
		console.log(i); // prints 1, 2, 3, 4, 5
	}
}

Here the function inner is called from a loop and iterator variable is printed from inside the function inner. It prints, as you guessed, 1, 2, 3, 4, 5. This is because the function inner get the value of i whatever it is when the function inner is invoked.

Loop, Closure and async invocation

Lets look at what happens if we invoke the function inner asynchronously

function outer(){
	for(var i=1; i<=5; i++){
		process.nextTick(inner); // We can use setImmediate or setTimeout in browser environment to get similar effect
	}
	
	function inner(){
		console.log(i); // prints 6, 6, 6, 6, 6
	}
}

You may not expect it to print 6, 6, 6, 6, 6 but that’s what it does. This is because the loop runs to completion in the current tick and the function inner is only invoked in some tick down the line. So, the value of i will be 6 when function inner is executed and it prints 6.

Now, what do you do if you want to print 1, 2, 3, 4, 5 instead of 6, 6, 6, 6, 6?
Well, it turns out, you can do this simply by creating new function to create a new variable scope. This is because, in JavaScript variables are scoped to function instead of block.

Note: Starting ES2015, you can use let instead of var to create block scope.

Here is the revised code:

function outer(){
	for(var i=1; i<=5; i++){
		process.nextTick((function(){
			var j = i;
			return inner;		
	
			function inner(){
				console.log(j); // prints 1, 2, 3, 4, 5
			}
		})());
	}
}

We have created a new scope and assigned value of i to j which is defined in the new scope. We have also moved the function inner to the new scope.
That’s it, now it prints what we want it to print.