Calling a function by variable

Question

I recently started to learn javascript by myself and I am reading a book called "Eloquent JavaScript". The following code is a sample script in the book which confused me:

function greaterThan(n) {
 return function(m) { return m > n; };
}
var greaterThan10 = greaterThan(10); 
console.log(greaterThan10(11));

Can someone please explain the logic of the last two lines? Does the greaterThan10 contain a truth value or it is a function?

• Sebastian Simon: greaterThan10 is a function.
• Sebastian Simon: Possible duplicate of How does this "higher-order functions" thing works in Javascript. You could’ve just Googled "greaterThan Eloquent JavaScript".

Answer 1

You define greaterThan10 on the second to last line:

var greaterThan10 = greaterThan(10); 

Whatever the greaterThan function returns in this case is what greaterThan10 will evaluate to.

On line 2 we see that greaterThan will return the following function expression:

function(m) { return m > n; }

After replacing the variable n with the value you passed we get this:

function(m) { return m > 10; }

Answer 2

It can look a little confusing at first, but just keep in mind that functions are objects in JavaScript.

greaterThan(n) is a function that returns an anonymous function with the definition:

function(m) { return m > n; }

Therefore, when we call greaterThan(10) we expect it to return an object that is in fact a function:

function(m) { return m > 10; }

Later on we just assign that object/function to a variable, and call it like we would call any function.

In short, just imagine that we had:

var greaterThan10 = function(m) { return m > 10; };

• jy l: Thanks for the explanation! It makes sense now:D