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
      

3. Built-in browser 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?");
         

4. 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);
      });
      

5. 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);
      });