JavaScript

This is based on w3schools JavaScript guide, and handpicked to examples and explanations which I think will greatly benefit for understanding the OSWE course.

Functions

In JavaScript there are 2 main types of functions you need to know. Declaration or function expression.

Declared functions

Declared functions are as follow. [ they are only for execution when invoked ] will not automatically execute. //semicolons are used to separate executable JavaScript statements.

function functionName(parameters){ 
  return a * b;
}

Function Expressions

Expression meaning. Reference: JavaScript Expressions and Statements [ Madu M ]

Function expressions can be stored in a variable and they are defined using an expression. This type of function is also called anonymous function (a function without a name)

var x = function(a,b){return a * b}; //ends with semicolon as it is part of executable statement
var z = x(5, 6); //wiill return 30 

The Function() Constructor

In previous examples, JavaScript functions are defined with the function keyword. But functions can also be built with function constructor called Function()

var myFunction = new Function("a", "b", "return a * b");
var myFunction2 = Function("c", "d", "return c * b"); //can omit the "new" keyword.
var x = myFunction(4, 3);
var y = myFunction2(4, 3);

console.log(x == y); //This will be true.

Function Hoisting

Hoisting is JavaScript's default behavior of moving all declarations to the top of the current scope. So the variable can be defined after being called. Functions can also be called first before declaring them.

myFunction(5);

function MyFunction(y){
  return y*y;
}

Self-Invoking Functions or Immediately Invoked Function Expression.

A self-invoking expression is invoked (started) automatically, without being called. Function expressions will execute automatically if the expression is followed by ().

*Note: You cannot self-invoke a function declaration. You have to add parentheses around the function to indicate that it is a function expression:

(function(){
 var x = "Hello";
})();

Arrow Functions

It allows a short syntax for writing function expressions. You don't need function keyword the return keyword and the curly brackets.

Callbacks

Callbacks are functions passed as an argument to another function.

function SayHi(name){
  console.log("Hello " + name);
}

function CallbackTest(firstname, lastname, callback){
  fullname = firstname + lastname
  callback(fullname)
} 

CallbackTest("Simple", "Guy", SayHi); //this will print Hello SimpleGuy

Instead of passing the name of a function, you can pass a whole function. [function expression]

setTimeout(function(){console.log("Hello!");}, 3000); //Returns Hello! after 3 seconds

Another example of opening a file. And only displaying it after the file has fully loaded.

function myDisplayer(some);
  console.log(some);

function getFile(callBack){
  let req = new XMLHttpRequest();
  req.open('GET', "longtext.txt");
  req.onload = function(){          //this is  an anonymous function inside an object
    if (req.status == 200){
      callBack(this.responseText); //you can use this keyword here as it is inside an object, and we know what object the key word is referring to
    } else{
      callBack("Error: " + req.status);
    }
  }
  req.send();
}