Higher order javascript functions

Question

can someone explain to me what is going on in the following code. The function is receiving n as parameter, so where is the m coming from? The whole code is confusing.. if someone can explain?

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

Answer 1

This is exhibiting a functional programming technique called currying. (related also to partial function appliction)

Greater than > usually takes 2 arguments (one on the left and one on the right). This is a way to feed one at a time.

It might be easier to see what is happening if you call it inline:

greaterThan(10)(11);

As you can see from the example above, the 10 gets passed in for the n parameter and then the 11 gets passed in for the m parameter.

The first application that passes the 10 outputs a function that looks like this:

function(m) { return m > 10; };

This is the first application in the partial application.

From there it is easy to see how the 11 is passed in to get the final result.

So, to break it down:

function greaterThan(n) {
   return function(m) { return m > n; };
}

//var greaterThan10 = greaterThan(10); -- is equivalent to:
var greaterThan10 = function(m) { return m > 10; };

console.log(greaterThan10(11));  //--> true

Answer 2

m is 11, passed in during the second call.

When you call greaterThan(10), it returns a new function that looks like:

function(m) {
    return m > 10;
}

which is then saved as greaterThan10. This is called currying.