callbacks.md

Higher-Order Functions

1. Takes a function as an input
2. returns a function as the output

---

Takes a function as an input

element.addEventListener( "click", function(){
  console.log("element clicked!");
  });

addEventListener() takes an anonymous function that logs "element clicked!" to the console.

Returns a function as the output

var add = function(num) {
  var num1 = num;

  return addToNum1 = function (num2){
    return num1 + num2;
  };
};

add returns addToNum1();

---

Callbacks

Functional Programming is about Using functions as units of abstraction instead of abstracting over data. A simple example is abstracting an if/else statement into a function.

Here is an example of an if/else statement abstracted into a function.

var ifElse = function( condition, isTrue, isFalse){

  if(condition){
    isTrue;
  } else {
    isFalse;
  }
};

ifElse( true, 
  function(){ console.log( true); },

  function(){ console.log( false); }

  );

To call ifElse(), you would need to pass in three arguments. First a Boolean, and then two functions If the conditional is true, you get the first function, and if it's false, you get the second.

Passing Arguments

functions passed as arguments can be invoked using their parameter name. These function can also have their own set of arguments passed to them when they are invoked.

var increment = function(n){ return n + 1; };
var square = function (n){ return n*n; };

var doMathSoIDontHaveTo = function(n, func){ return func(n); };

doMathSoIDontHaveTo( 5, square);

doMathSoIDontHaveTo( 4, increment);

---

var funCaller = function(func, arg) {
    return func(arg);
};

var firstVal = function(arr, func) {
    func(arr[0], 0, arr);
};

change firstVal to work not only with arrays, but also objects. Since objects are not ordered, you can use any key-value pair on the subject

firstVal = function(list, func){
  if( Array.isArray( list )){
    func( arr[ 0], 0, arr );
  } else {
    for(var k in list){
      return func( list[k], k, list);
    }
  }
}

Using Object.keys: This returns an array of all keys in an object.

firstVal = function(list, func){
  if( Array.isArray( list )){
    func( arr[ 0], 0, arr );
  } else {
    var propArr = Object.keys( list );
    func( list [ propArr[0] ], propArr[0], list )