Creating Unchanging Variables With const

The first method you should consider when creating a variable is the const keyword. 0:00 This is often the best choice for declaring variables. 0:06 Const is short for constant, meaning that the value of the variable shouldn't change 0:09 it should remain constant. 0:14 For example const pi is equal to 3.14159. 0:16 Const let's you protects variables from being overwritten by any stray assignments 0:21 in your code. 0:26 In other words, once you create a constant, 0:27 you can't give it another value. 0:29 Let me show you what I mean. 0:32 If you want to follow along you can open up the workspace on this page. 0:34 Open up pi.html, and as you can see we've assigned 0:38 a value of 3.1459 to the constant Pi. 0:45 Click on preview and navigate to pi.html. 0:51 Open up the developer tools and head over to the JavaScript console. 0:56 When we type pi, the value is logged. 1:01 Try to assign another value to pi. 1:05 We see we get an error type error assignment to a constant variable. 1:11 You will use constants frequently. 1:17 In fact, it should be your first choice when declaring variables. 1:19 You only use them to store numbers that shouldn't change while your program runs. 1:22 For example, tax rates, dimensions of user interface components, and 1:27 product prices are good examples. 1:31 And you'll use constants when your select an elements on a page, or 1:33 when you are assigning a function to a variable. 1:38 Any value can be assigned to a constant. 1:41 However, for a variable whose value will change during the life of the program, 1:43 const are a bad choice. 1:48 For example you wouldn't want to const to store a score of a game 1:50 since the score changes as the player plays the game. 1:54 In that case, you'll use the let keyword. 1:59 I'll show show you how that works in another video. 2:01 As I mentioned, constants help you fix problems 2:05 that can easily crop up in your JavaScript programs. 2:08 Let's take a look at a common problem that you'll run into. 2:12 If you want to follow along, open up the name.html file in these video's workspace. 2:15 Here's a simple script with the variable name. 2:22 I'll preview this file and check in the console. 2:25 Notice that the console says Andrew. 2:32 That's the value of the name variable defined on line one. 2:36 The console then prints out Joe Craft, 2:40 which is logged by the create full name function. 2:44 Now that the programs run, what's the value of the name variable? 2:49 I'll just check that out in the console by writing name. 2:54 It's now Joe Craft. 2:58 Andrew is gone. 3:00 The variable has been overwritten by the function. 3:02 This is a simple scope issue. 3:06 The function is overwriting the name variable in the global scope. 3:09 It's a problem that can pop up and 3:14 can be difficult to catch, however const will help you spot the mistake. 3:16 Now when I preview this, the console spits out an error. 3:24 You can't change a value of a constant. 3:33 Because I used const, I can see that the simple mistake I made. 3:36 And I can go back into my script and fix it 3:40 Simply adding const before the name within the function keeps 3:48 that variable locked to the scope of the function. 3:52 It's no longer trying to reassign the name variable. 3:55 I'll preview this script one more time. 3:59 And no errors. 4:04 And I can log out name, and it's still Andrew. 4:05 Hooray, I'm not gone. 4:08