samnags/javascript-intro-to-functions-lab-intro-to-js-test-batch

JavaScript Intro to Functions Lab

Objectives

1. Practice writing functions
2. Explain basics of working with strings
3. Explain the difference between return and logging
4. Practice using return and console.log()

Introduction

Welcome to the JavaScript functions lab! You'll notice a few new things in this lesson that we haven't encountered before. Don't worry, we'll walk you through them.

Even if you've walked through some of this material before, it's a good idea to review as we code-along — we're writing functions now, after all.

Code-along

For now, open up index.js in your text editor. You should see, well, nothing. We'll fix that soon.

Now upon up test/index-test.js. Hey, there's something! What's all of this stuff doing?

At the very top of the file, you'll see

const expect = require('expect')
const fs = require('fs')
const jsdom = require('mocha-jsdom')
const path = require('path')

This might be a bit bewildering, but all we're doing is referencing different libraries that help us run your tests. A library is code that someone else (usually multiple someone elses) wrote for our use. Note that require won't work out of the box in the browser. We're actually running our tests in a different environment. (Remember the sandbox analogy from earlier? It's just like that.)

A little farther down the page, you'll see

describe('index', () => {
  // there's stuff in here, too
})

describe is a function provided by our test runner (in this case, we're using Mocha) — it's basically a container for our tests.

Then we have

jsdom({
    src: fs.readFileSync(path.resolve(__dirname, '..', 'index.js'), 'utf-8')
})

This line reads index.js (remember how we said we'd modify that?) and adds its code to the execution environment.

After that, there's another describe():

describe('shout(string)', () => {
  it('receives one argument and returns it in all caps', () => {
    // we'll get to this in a sec
  })
})

These internal describe() calls are used for describing the functions that you're going to write. In this case, the test is saying, "Okay, I think there's going to be a function called shout, and it should take one argument (it doesn't actually matter what the argument is called, but string, is nice and specific, don't you think?). It should return that argument in all caps.

Finally, we have

expect(shout('hello')).to.equal('HELLO')

which says that it expects a call to shout() with the string 'hello' will equal the string 'HELLO'. This is the actual test — otherwise called a spec, expectation, or assertion — for this function. We can have more than one test per function, but let's start with this one.

Running the Tests

To run the tests, run learn test in the terminal in your Learn IDE. The first output you'll see will look like

Hm, that's kind of disappointing. Let's see if we can get that first test to pass. Open up index.js.

When we write our code, we follow the guidance of the tests. Remember the line, describe('shout(string)', () => { ... }). Well, we know that we need a function called shout that accepts an argument — let's add that first. In index.js:

function shout(string) {
}

And what should that function do? Well, the it() description tells us that it "receives one argument and returns it in all caps".

Okay, so with that information, we know that our function should look like

function shout(string) {
  return string
}

But how do we make string all caps? JavaScript has a method for that! It's called toUpperCase(). We can call it on any string:

'Hello!'.toUpperCase() // 'HELLO!'

So let's try it with our shout() function:

function shout(string) {
  return string.toUpperCase()
}

And run our tests again:

learn

Hey! We got one to pass!

Your turn

Now it's your turn to get the rest of the tests to pass. Note that some of them require you to use console.log() instead of return — follow the guidance of the tests!

Oh, lastly: just like .toUpperCase() changes any string to all uppercase in JavaScript, .toLowerCase() (e.g., 'HELLO'.toLowerCase()) changesany string to all lowercase.

Good luck! When you're finished, be sure to run learn submit!