Assertions

Loading "Imperative Vs Declarative Assertions"

Okay, let's apply that test structure to our test for the greet function.

Test structure

First, let's take a look at what test phases we already have.

Setup

We don't have any special setup to test the greet function. The setup phase is optional and you will often find it missing in unit tests (testing isolated pieces of code like we have over here).

Action

Our action for this test is to call the greet function with the 'John' string as the argument.

let message = greet('John')

Assertion

And our assertion is a simple if statement that compares the actual message with the message we expect to be returned ('Hello, John!').

// 🐨 Replace these if/throw blocks with the "expect" function
// you will introduce later down this file.
// 💰 expect(greet('John')).toBe('Hello, John!')
if (message !== 'Hello, John!') {
	throw new Error(

The problem

Our assertion has two problems.

First, it's imperative. We instruct the code how to do things instead of declaring what we want to do.

Second, it's repetitive. This may not be as evident right now, but consider what happens if we have to test a new function called congratulate:

function greet(name) {
	return `Hello, ${name}!`
}

function congratulate(name) {
	return `Congrats, ${name}!`
}

let message = greet('John')

if (message !== 'Hello, John!') {
	throw new Error(
		`Expected message to equal to "Hello, John!" but got "${message}"`,
	)
}

message = congratulate('Sarah')
if (message !== 'Congrats, Sarah!') {
	throw new Error(
		`Expected message to equal to "Congrats, Sarah!" but got "${message}"`,
	)
}

It would be great to abstract that logic into a declarative function called expect. With that function, these two tests would look like this:

function greet(name) {
	return `Hello, ${name}!`
}

function congratulate(name) {
	return `Congrats, ${name}!`
}

expect(greet('John')).toBe('Hello, John!')
expect(congratulate('Sarah')).toBe('Congrats, Sarah!')

Preferring declarative APIs when testing often leads to more readable and manageable tests.

👨‍💼 Your task right now is to implement this expect function and refactor the existing tests to use it. Feel free to use this outline for reference:

function expect(actual) {
	return {
		toBe(expected) {
			// Compare the actual to the expected
			// and throw if they don't match.
		},
	}
}

02. Test Structure/01. Assertions (💪 problem)