Yes, absolutely.

It may not be "Simple", but function composition certainly is "Fundamental".

Look at it this way: "1243".split("").sort().join("") is also function composition, just with a different syntax.

( That could have been compose( join(""), sort(), split("") )( "1243" ). Mind that the arguments to compose are functions. )

Did you read eloquent javascript? The main thing that I think would be confusing is scope and closures, so perhaps looking into those two topics would be helpful.

function mainScope (args) { //main function
var newfunction = function(){ //still has access to variables accessible in mainScope
return args; //can manipulate args as expected
};
return args;
}

function secondaryScope (args) { //new function
newFunction(); //would throw error
}

Check out ternary operators :)

You should post this in the discuss thread of the kata you are having trouble with. You will see discuss with some speech bubbles just below the katas title. I believe this is the kata you are talking about. function within a function
There are lots of things you can do with functions in javascript. In Javascript functions are actually just a special type of object so they can be returned or even passed around.
so a function that returns another function would look something like this

function a(n){
  return function(){
    ///do something with the argument n
  }
}

var b = a(9)
// then b is a function that does something with 9
b()  // is the same as a(9)()

I hope that helps. If you are still stuck or just want to learn more about functions in functions here are some things to research on Google

• Javascript closures
• Immediately Invoked Function Expressions(IIFE)
• Javascript scope