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
}