- Learn about testing in JavaScript
- Run JavaScript tests
- Learn how to read test results
Many of the labs you will complete in this program use tests. Passing the tests verifies that the code you write behaves as desired and produces the expected results. But writing tests is also a way to provide specifics about exactly how the code should function. In a common development strategy known as test-driven development (or TDD), programmers first write the test for a specific function of the code then write the code to make the tests pass. TDD is considered the most reliable methodology for delivering quality code.
What this means is that the results of running the tests will be an important tool in figuring out how to get those tests passing. Code testing can feel like an abstract concept at first, but it's worth starting to learn how they work. When you're having difficulty passing a test, being able to read and understand the test output — and the tests themselves — can be an invaluable skill.
The lesson is set up as an assignment (lab), so you'll first need to fork and clone it to your local environment.
If you haven't already, fork and clone this lesson into your local environment.
Navigate into its directory in the terminal, then run code . to open the files
in Visual Studio Code. (If you are using a different text editor, the command
will be different.)
Open up index.js in your code editor. You are going to see mostly familiar
things:
const name = "Joe";
const height = 74;
const message = `${name} is ${height} inches tall`;
module.exports = { name, height, message };This should all look familiar except for that last line. You don't need to worry about it for now — just know that line of code makes the variables available to the test file.
Take a look at the message variable:
const message = `${name} is ${height} inches tall`;We can use console.log to take a look at the value of the message variable.
To do that, first type console.log(message); on the last line of index.js
and save the file. Next, navigate to the terminal, and type the following
command in the command line and hit enter (be sure you're still in the lab's
directory):
$ node index.jsThe node command executes the code in whatever file you specify (in this
case, index.js). You should see "Joe is 74 inches tall" logged in the
terminal.
Top Tip:
console.logis one of the debugging tools you can use as you're writing your code. Logging a variable and executing the code will allow you to verify that the value of the variable is what you're expecting.
In the line of code above, we are using string interpolation to inject the
values of the name and height variables into the message. Recall that, for
this to work, you have to wrap the entire string in backticks and wrap the
variables themselves in ${}. If you'd like a refresher, try leaving out the
${}s or switching to a different type of quotes and run your code again to see
what the value of message is. The backticks and the ${} tell Javascript to
grab the value inside the variable, not just that variable name.
Go ahead and delete the console.log from index.js before moving on.
We have our code, now let's take a look at the tests. They are located in the
test folder inside a file named index-test.js.
const { name, height, message } = require("../index.js");
describe("what-is-a-test", () => {
describe("Name", () => {
it('returns "Susan"', () => {
expect(name).toEqual("Susan");
});
});
describe("Height", () => {
it("is less than 40", () => {
expect(height).toBeLessThan(40);
});
});
describe("Message", () => {
it("gives the name and height", () => {
expect(message).toInclude(name);
expect(message).toInclude(height);
});
});
});Important: You should never need to make changes to test files unless a lab's instructions specifically tell you to do so.
In the first line, we're enabling the tests to access the variables in
index.js. You don't need to worry about exactly how this works at this point —
just know that the module.exports and require keywords allow us to access
variables written in the index.js file from within the test file.
Next, note that the test code consists of three individual tests (each starting
with describe) nested inside a block for the tests as a whole (also starting
with describe).
The first grouping is testing our name variable:
describe("Name", () => {
it('returns "Susan"', () => {
expect(name).toEqual("Susan");
});
});Take a look at the line that begins with expect. If we read it out loud, we
get "Expect name to equal Susan". That's exactly what it's saying! If we
continue down to the Height section you'll see this code:
describe("Height", () => {
it("is less than 40", () => {
expect(height).toBeLessThan(40);
});
});Again, reading the line starting with expect out loud, we get "Expect height
to be less than 40." Again, this is just what the test is checking. Let's look
at the final one:
describe("Message", () => {
it("gives the name and height", () => {
expect(message).toInclude(name);
expect(message).toInclude(height);
});
});This one has two expect statements. If you read them out as English you'll
discover that the tests expect the value of index.message to include both
index.name and index.height.
OK great. Now that you understand what the tests are saying, it's time to run them.
To run the tests, make sure you're inside the lab's directory in the terminal,
then run learn test. Recall that this command first installs the lab's
dependencies, then shows the results of running the tests.
What exactly do we mean by installing dependencies? Open the
package.jsonfile and scroll down to the bottom. You'll see a list of 'DevDependencies'. What's listed here are JavaScript packages: files or sets of files full of existing, reusable code. They are designed to be shared, allowing many developers to use the same code in their own projects. The packages you see listed inpackage.jsonmake it possible to run the lab's tests. In order to use the packages, we have to install them. One of the thingslearn testdoes for us is runnpm install, which is the command that installs the dependencies.
The next step is learning how to read the results that the tests give you.
The first time you run learn test, you should see something that looks like
this:
> [email protected] test
> mocha --timeout 5000 -R mocha-multi --reporter-options spec=-,json=.results.json
what-is-a-test
Name
1) returns "Susan"
Height
2) is less than 40
Message
✓ gives the name and height
1 passing (552ms)
2 failing
1) what-is-a-test
Name
returns "Susan":
Error: Expected 'Joe' to equal 'Susan'
+ expected - actual
-Joe
+Susan
at assert (node_modules/expect/lib/assert.js:29:9)
at Expectation.toEqual (node_modules/expect/lib/Expectation.js:81:30)
at Context.<anonymous> (test/index-test.js:6:26)
at processImmediate (internal/timers.js:461:21)
2) what-is-a-test
Height
is less than 40:
Error: Expected 74 to be less than 40
at assert (node_modules/expect/lib/assert.js:29:9)
at Expectation.toBeLessThan (node_modules/expect/lib/Expectation.js:156:28)
at Context.<anonymous> (test/index-test.js:13:28)
at processImmediate (internal/timers.js:461:21)
npm ERR! Test failed. See above for more details.Note: If you also get an error that ends with "unexpected character (after ) at line 1, column 1 [parse.c:769] (Oj::ParseError)", go back to the
index.jsfile and remove theconsole.logwe added earlier, then runlearn testagain.
Let's break this down a bit. If you look about a third of the way down in the output, you'll see a summary of how the tests went:
1 passing (552ms)
2 failingGreat! We've already got one test passing! Now let's see how we failed the other two tests.
1) what-is-a-test
Name
returns "Susan":
Error: Expected 'Joe' to equal 'Susan'
+ expected - actual
-Joe
+Susan
at assert (node_modules/expect/lib/assert.js:29:9)
at Expectation.toEqual (node_modules/expect/lib/Expectation.js:81:30)
at Context.<anonymous> (test/index-test.js:6:26)
at processImmediate (internal/timers.js:461:21)
2) what-is-a-test
Height
is less than 40:
Error: Expected 74 to be less than 40
at assert (node_modules/expect/lib/assert.js:29:9)
at Expectation.toBeLessThan (node_modules/expect/lib/Expectation.js:156:28)
at Context.<anonymous> (test/index-test.js:13:28)
at processImmediate (internal/timers.js:461:21)While there is no hard and fast rule, and there will be exceptions, it is most often best to address your test errors in order. So let's take a look at our first error:
1) what-is-a-test
Name
returns "Susan":
Error: Expected 'Joe' to equal 'Susan'
+ expected - actual
-Joe
+Susan
at assert (node_modules/expect/lib/assert.js:29:9)
at Expectation.toEqual (node_modules/expect/lib/Expectation.js:81:30)
at Context.<anonymous> (test/index-test.js:6:26)
at processImmediate (internal/timers.js:456:21)Here is the specific error:
Error: Expected 'Joe' to equal 'Susan'
+ expected - actual
-Joe
+SusanIt tells us what the test is expecting (Expected 'Joe' to equal 'Susan') and
then gives us details about the expected and actual values. This shows you
exactly how the value your code is returning (the actual value) differs from
what the test is looking for. Make sure you understand what this is telling you
— it will come in handy in later labs!
This error makes sense because we have the name variable set equal to "Joe" in
our index.js file. Let's change that line of code to set name equal to
"Susan" instead. Run the tests again by typing learn test in the terminal's
command line, and you should see that we are now passing 2 of the 3 tests!
what-is-a-test
Name
✓ returns "Susan"
Height
1) is less than 40
Message
✓ gives the name and height
2 passing (736ms)
1 failing
1) what-is-a-test
Height
is less than 40:
Error: Expected 74 to be less than 40
at assert (node_modules/expect/lib/assert.js:29:9)
at Expectation.toBeLessThan (node_modules/expect/lib/Expectation.js:156:28)
at Context.<anonymous> (test/index-test.js:13:28)
at processImmediate (internal/timers.js:461:21)Woot! You passed another one. Now go ahead and try to pass the remaining test on your own.
While you are solving the other tests you may come across a few errors. Let's go over some common ones:
ReferenceError: name is not definedThat one says that the name variable is not defined. That makes no sense! We
initialized the name variable in index.js! What that actually means is that
the test couldn't find the variable name. You'll get this error if the name of
one of your variables is different than the test is expecting. Check to make
sure you used the correct variable names and look carefully for typos.
/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/index.js:1
cnst name = "Susan";
^^^^
SyntaxError: Unexpected identifier
at wrapSafe (internal/modules/cjs/loader.js:979:16)
at Module._compile (internal/modules/cjs/loader.js:1027:27)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
at Module.load (internal/modules/cjs/loader.js:928:32)
at Function.Module._load (internal/modules/cjs/loader.js:769:14)
at Module.require (internal/modules/cjs/loader.js:952:19)
at require (internal/modules/cjs/helpers.js:88:18)
at Object.<anonymous> (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/test/index-test.js:1:15)
at Module._compile (internal/modules/cjs/loader.js:1063:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
at Module.load (internal/modules/cjs/loader.js:928:32)
at Function.Module._load (internal/modules/cjs/loader.js:769:14)
at Module.require (internal/modules/cjs/loader.js:952:19)
at require (internal/modules/cjs/helpers.js:88:18)
at /Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/lib/mocha.js:311:36
at Array.forEach (<anonymous>)
at Mocha.loadFiles (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/lib/mocha.js:308:14)
at Mocha.run (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/lib/mocha.js:849:10)
at Object.exports.singleRun (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/lib/cli/run-helpers.js:108:16)
at exports.runMocha (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/lib/cli/run-helpers.js:143:13)
at Object.exports.handler (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/lib/cli/run.js:305:3)
at Object.runCommand (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/yargs/lib/command.js:242:26)
at Object.parseArgs [as _parseArgs] (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/yargs/yargs.js:1104:24)
at Object.parse (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/yargs/yargs.js:566:25)
at Object.exports.main (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/lib/cli/cli.js:68:6)
at Object.<anonymous> (/Users/lizburton_fs/Development/code/phase-0-pac-3-what-is-a-test-lab/node_modules/mocha/bin/mocha:133:29)
at Module._compile (internal/modules/cjs/loader.js:1063:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
at Module.load (internal/modules/cjs/loader.js:928:32)
at Function.Module._load (internal/modules/cjs/loader.js:769:14)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12)
at internal/main/run_main_module.js:17:47
npm ERR! Test failed. See above for more details.Whoa! So many words that make no sense. Don't worry though. The most important
line is the SyntaxError: Unexpected identifier line. What that means is you
have some sort of typo or syntax mistake. It could be a HUGE variety of things
but usually, JS will try and give you a hint. This time it's pointing to the
cnst name = "Susan" line of code. Take a look and read very carefully:
const is misspelled. Whoops! Once we fix that everything will work.
One note on this type of error is that it is sort of a catch-all. Tons and tons of problems end in that sort of error message. Whenever you see it, be sure to read over your code with a fine-toothed comb... and you'll find the problem!
On the second test, there is a chance you might see the following error:
1) what-is-a-test
Height
is less than 40:
Error: The "actual" argument in expect(actual).toBeLessThan() must be a number
at assert (node_modules/expect/lib/assert.js:29:9)
at Expectation.toBeLessThan (node_modules/expect/lib/Expectation.js:156:28)
at Context.<anonymous> (test/index-test.js:13:28)
at processImmediate (internal/timers.js:456:21)This error is slightly different than the last two. In this case, the test is
giving us a unique message because it recognizes a problem. If we look at this
test in test/index-test.js, we see this:
describe("Height", () => {
it("is less than 40", () => {
expect(height).toBeLessThan(40);
});
});We can see that the word "actual" in this case is referring to the height
variable. The error message is telling us that height must be a number. If
you're seeing this, make sure that you have set the height variable to a
number that's less than 40 (e.g. 39), not a string ("39"). The test
will interpret the value as a string due to the quotation marks wrapping it.
In this lab, we only had three tests to pass, but as you continue through the curriculum you will encounter labs with many more tests. You can imagine that the test output could get very long, making it more difficult to focus in on how to fix a particular error.
To help with this issue, there is some very simple setup you can put in place in Mocha that will cause the tests to stop as soon as the first failing test is encountered.
To implement this, open up the package.json file and find the test script. It
should look something like this:
"test": "mocha --timeout 5000 -R mocha-multi --reporter-options spec=-,json=.results.json"Add the --bail flag to the end of the line, inside the quotes:
"test": "mocha --timeout 5000 -R mocha-multi --reporter-options spec=-,json=.results.json --bail"That's it!
Once you've got all your tests passing, you're ready to move on. Congratulations! You've solved your first JavaScript tests!