JavaScript Functions

1. Overview
   1. Functions may be defined before or after a call to the function
   2. Function parameters are always passed by value
   3. Function that does not return a value (like C++ void function)

      function sayHello(name) {
      	document.writeln("Hello, " + name);
      }
      

   4. Function that returns a value

      function max(x, y) {
      	if (x < y)
      		return y;
      	else
      		return x;
      }
      

2. Scoping
   1. Local variables are scoped to the function they are declared in (only accessible within the function)
   2. Global variables are declared outside a function and have global scope (accessible anywhere)
   3. Example:

      let x = 3;
      
      function test() {
      	// y is local
      	let y = 10;
      	document.writeln("y is " + y);
      	
      	// x is global
      	document.writeln("x is " + x);
      }
      

   4. Default parameter values

      // z is 0 if not supplied
      function sum(x, y, z = 0) {	
      	return x + y + z;
      }
      
      total = sum(1, 2);     // 3
      total = sum(1, 2, 4);  // 7
      

   5. Built-in functions
      1. Global functions (sample)
         1. parseFloat("s") - Converts the beginning of a string to a floating-point value; returns NaN if unsuccessful

            let x = parseFloat("1.23");   // 1.23
            

         2. parseInt("s") - Converts the beginning of a string to an integer value; returns NaN if unsuccessful

            let x = parseInt("66");   // 66
            

         3. isNaN("s") - Returns true if the argument is not a number, false otherwise

            if (isNaN("dog")) {
            	console.log("'dog' is not a number.");
            }
            
            // Careful... empty string or string with all spaces is NOT NaN!
            if (!isNaN("")) {
                console.log("Empty string is a number!");
            }
            

         4. isFinite(value) - Returns true if the argument is a number in the range that JavaScript supports, false otherwise

            isFinite(123)           // true
            isFinite(6.5)           // true
            isFinite(false)         // true (because false = 0)
            isFinite("dog")         // false
            

      2. Dialog boxes
         1. alert(s) - Alert dialog box with OK button
            
            

            alert("This is an alert.");
            

         2. prompt(s) - Dialog box that prompts the user for a single line of text; returns the text or null if Cancel is pressed
            
            

            let firstName = prompt("Enter your name:");
            

         3. confirm(s) - Dialog box that asks a question; returns true if OK is pressed, false otherwise
            
            

            let answer = confirm("Are you sure?");
            

   6. Anonymous functions
      1. An anonymous function is a function that doesn't have a name

         // Assign an anonymous function to a variable
         let hello = function() { alert('Hello'); }
         
         // Variable acts like a function
         hello();
         

      2. Pass anonymous functions to functions expecting function references

         let nums = [8, 2, -1, 5];
         
         // Sort in descending order
         nums.sort(function(a, b) {
             return b - a;
         });
         
         // Display the even numbers in the array
         nums.forEach(function(n) {
         	if (n % 2 == 0)
         		console.log(n);
         });
         

   7. Arrow functions (introduced in ES6)
      1. arrow functions are a replacement for wordy anonymous functions
      2. Basic syntax

         (param1, param2, …, paramN) => { statements }  // No return value
         (param1, param2, …, paramN) => expression      // same as: => { return exp; }
         
         // Parentheses are optional when there's only one parameter:
         (singleParam) => { statements }
         singleParam => { statements }
         

      3. Example arrow function that doesn't return a value

         let hello = name => console.log("Hello, " + name);
         hello("Bob"); 
         
         // Same as
         let hello = function(name) {
         	console.log("Hello, " + name);
         };
         

      4. Example arrow functions that return values

         let add = () => 1 + 2;
         let num = add();  // 3
         
         // Same as
         let add = function() {
         	return 1 + 2;
         };
         
         let subtract = (x, y) => x - y;
         let num = subtract(4, 3);   // 1
         
         // Same as
         let subtract = function(x, y) {
         	return x - y;
         };
         

      5. Use anytime you need an anonymous function

         let nums = [8, 2, -1, 5];
         
         // Sort in descending order
         nums.sort((a, b) => b - a);
         
         // Display the even numbers in the array
         nums.forEach(n => {
         	if (n % 2 == 0)
         		console.log(n);
         });