To show you how BDD and the red green refactor cycle helps us, let's imagine we 0:00 have an application pulling movie titles from different sites on the Internet. 0:05 Because the information comes from multiple websites 0:09 we can't be sure that the titles coming in have the style we want. 0:12 We want the titles in our database to be nicely title cased. 0:16 Which means that the first letter of every word is capitalized except for 0:20 certain less important words like the, and, of and so on. 0:23 I've linked to some examples of title case in the teacher's notes as well. 0:27 So, let's try writing a function that can handle this title casing for 0:32 us using BDD principles. 0:35 First, let's install a testing tool called cahi. 0:38 Chai is what's called an expectation library. 0:42 Chai include special functions that throw an error when an expectation is not met. 0:45 For now we can use chai functions to write our BDD outline for a small function. 0:50 You can follow along in workspaces or 0:54 in your own command line terminal on your computer. 0:56 Okay, so, first we need to install chai for our project. 0:59 So in my workspace console I'll type, npm install chai, 1:04 followed by --save -dev. 1:09 So I'm using the --save-dev flag here for my npm installation instead 1:13 of just save because chai won't be a dependency for our working code. 1:18 We don't actually need chai or 1:22 any of our testing tools in order to run our application. 1:24 We just need to for our development process. 1:27 Now if you forget how to set this up, be sure to check the teacher's notes for 1:29 a link to chai's documentation and installation instructions. 1:33 Next, I'll create a new JavaScript file by going over to the console and 1:38 typing touchtextutilities.js. 1:42 I'm calling my JavaScript file text utilities.js because the title case 1:49 function that we're going to write is just a tool for manipulating text. 1:53 Once again, you can refresh your sidebar to see the new textutilities.js file. 1:57 So, when I open our textutilities file, 2:08 the first thing I'm gonna do is import the stuff we want from the cai library. 2:10 We specifically want to use the expect method. 2:15 Check the teacher’s notes to read about other methods you might prefer, but 2:19 I’m just going to use expect for the rest of the course. 2:22 So, to import expect from the chai library, 2:26 I'll type var expect = .expect;. 2:31 Remember, to import stuff, 2:37 we write require followed by the name of our library as a string. 2:39 So, inside the quotes, I'll type, chai. 2:45 And now Node will know to go look for 2:49 a package with that name in our node_modules directory. 2:51 Now you might not have seen this style of require statement before. 2:57 But don't worry, since I don't want to use everything in chai, 3:01 I'm just saving the expect method to a variable and throwing the rest away. 3:04 So, writing it this way is a more efficient way of writing it like this. 3:10 So var chai = require('chai') followed by var expect = chai.expect. 3:16 Now that we have the expect function in our file, 3:23 we can use it to write some expectations as our outline. 3:26 To use it, we write expect and pass in a value we want to use for comparison. 3:30 Then we chain some special child chai methods to tell chai 3:37 what we expect that value to be. 3:40 So for example, we can say expect(true).to.be.false. 3:42 So, expect(true).to.be.false. 3:50 Now this is obviously going to fail because true does not equal false. 3:52 So, if I save this and run our test file in the console using node, 3:56 followed by the name or a file, which is textUtilities.js. 4:00 Once I hit Enter we see that I get an error. 4:07 It says, AssertionError: expected true to be false. 4:10 Cool, so now I can go back to our code and fix this example so 4:13 that it's passing by switching false with true. 4:18 And now if I run node textUtilities.js again, we don't see any output at all. 4:24 So, there were no errors. 4:31 We're going to see a lot more useful output when we combine chai 4:32 with our unit testing framework Mocha in the next stage. 4:36 But for now, this will be our green. 4:39 All right, so now let's write our first expectation for the titleCase function. 4:47 So I'll start by writing expect(titleCase. 4:54 And then pass in an example of a movie title or 5:00 application might be sent from the Internet. 5:03 So I'm going to type, the great mouse detective. 5:06 Notice the title has no capital letters. 5:16 I'm just making this up as an example for me to test against. 5:19 I could put anything I want to in here. 5:23 This is just acting as a guide for me as I write this function. 5:25 So, what do I expect the output of our titleCase function 5:29 with the great mouse detective to be? 5:32 Well, I definitely know that I should get a string back, right? 5:36 A string goes in and a string comes out. 5:39 So, let's start with that. 5:40 So I'll say, 5:42 .to.be.a('string'). 5:45 So, expect titleCase('the great mouse detective') to be a string. 5:51 With my first expectation written I could go run our 5:56 file again in the console with our command node textutilities.js. 5:59 And, this gives me an error now. 6:04 It says, ReferenceError: titleCase is not defined. 6:06 And that's right because I haven't written the titleCase function yet. 6:11 Now, we should fix just one error at a time. 6:15 So, let's make this go away. 6:18 Back in my textutilities.j sfile, right above the test we just wrote, 6:20 I'll write a new function named titleCase. 6:25 It'll say function tittleCase. 6:28 So now, when I save the file and run the file again in the console, 6:39 I see that I stop getting a reference error. 6:44 Instead, I get an error that says, 6:49 AssertionError: expected undefined to be a string. 6:51 Perfect, it looks like we're making progress. 6:55 Now, at this point I know that titleCase is going to accept 6:57 a string as an argument. 7:00 And it's going to do some stuff to it and give me back a new string. 7:02 So, I can get our current test to pass by doing just that much. 7:06 So, if I let the titleCase function accept a title and 7:11 then spit it back out by typing, return title. 7:15 Once I save my JavaScript file and run the test again in the console. 7:18 Great, we can see that it meets our expectations, 7:25 but our expectations aren't really accurate yet, right? 7:29 And this is an important lesson about testing. 7:33 We can pass all our tests even if nothing works correctly if our test code doesn't 7:35 really test what we want it to. 7:40 So, in the next video let's add some more expectations to flush out our outline. 7:42 We'll be talking a lot about how to do this during the course. 7:47 For now, I've linked to an article in the teacher's notes about 7:50 ways we can make our expectations really informative. 7:53