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JS Functions


Functions are first–class citizens in JS, meaning that they can be passed around like any other type of data, eg, variables.

Declared function

Declared functions build in memory immediately when the program loads.

Basic syntax

function sumOfCubes(a, b) {
  return a*a*a + b*b*b;

sumOfCubes(4, 9); // returns 793

// Parameters can also be expressions
sumOfCubes(1+2, 3+5); // returns 539

console.log(myFunction); will print function body

A more complex declared function

function countE() {
  var phrase = prompt("Which phrase would you like to examine?");
  if ( typeof(phrase) != "string" ) {
    alert("That's not a valid entry!");
    return false;
  } else {
    var eCount = 0;
    for var index = 0; index < phrase.length; index++ {
      if (prase.charAt(index) == 'e' || phrase.charAt(index) == 'e') {
    alert("There are " + eCount + " E's in \"" + phrase + "\".");
    return true;

Local vs Global Scope

Inside functions the scope is local. Outside of the function the scope is global meaning that variable are potentially accessible everywhere

var x = 6; // x is global
var y = 4; // y is global

function add (a, b) { // a and b are local to the function
  var x = a + b;  // x is local b/c declaring w/ var inside a function doesn't
  return x;       // modify same-name variable outside the function.

function add (a, b) {
  x = a + b;  // if x is not declared with var, it will shadow the same-name 
  return x;   // variable from the nearest external scope.

Function expressions

Function expressions load when it's line of code is reached.

Anonymous function expression

var myFunction = function() {
}; // semi-colon on final bracket

Function expression as direct parameter

The built-in map function will loop through array elements. The following example will take an array of arrays and convert it to 1 dimensional array of strings

var peopleNames = [ ["Jonas", "Rogers"], 
                    ["Mel", "Gibson"], 
                    ["Lucy", "Liu"], 
                    ["Edward", "Scissorhands"] ];

var modifiedNames ={
	return arrayCell[0] + " " + arrayCell[1];

--> ["Jonas Rogers", "Mel Gibson", "Lucy Liu", "Edward Scissorhands"] 
Returned Functions

Since a function can be an expression, it can be returned as a value.

function aGoodThing ( chickenShit ) {
	return function ( chickenShit ) {
		alert ("The chicken salad is good today.");
Immediate Invocation

Javascript functions can be invoked without the need to store in a variable. Behold.

function makeSandwich (bread, meat, cheese)(); // Need parameter parenthesis and semi–colon to execute
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