Get Your Chai Fix and Learn the Art of Assertions at the Same Time
Tags: unittesting, integrationtesting, testing, library
Published: 2023-01-06 01:21:03

---
title: Get Your Chai Fix and Learn the Art of Assertions at the Same Time
published: true
date: 2023-01-05 17:16:00 UTC
tags: UnitTesting,IntegrationTesting,Testing,Library
canonical_url: https://techdevblog.io/get-your-chai-fix-and-learn-the-art-of-assertions-at-the-same-time/
---

 ![Get Your Chai Fix and Learn the Art of Assertions at the Same Time](https://techdevblog.io/content/images/2023/01/image_7-1.png)

Hey there! Are you tired of writing tedious and repetitive test cases for your JavaScript code? Look no further, because [Chai](https://www.chaijs.com) is here to save the day!

[Chai](https://www.chaijs.com) is a popular assertion library for [Node.js](https://nodejs.org/en/) and the browser that makes it easy to write powerful and elegant test cases. It provides a range of functions that allow you to make a variety of assertions about your code, from simple true/false statements to more complex comparisons.

In this tutorial, we'll go over the basics of using [Chai](https://www.chaijs.com) to write test cases for your JavaScript code. By the end, you'll be a [Chai](https://www.chaijs.com) pro and your test suite will be running smoothly in no time!

## Setting up [Chai](https://www.chaijs.com)

Before we dive into writing test cases with [Chai](https://www.chaijs.com), let's first make sure it's set up in your project.

If you're using [Node.js](https://nodejs.org/en/), you can install [Chai](https://www.chaijs.com) using npm:

```
npm install --save-dev chai

```

If you're using a browser, you can include [Chai](https://www.chaijs.com) in your HTML file using a `script` tag:

```HTML
<script src="https://unpkg.com/chai/chai.js"></script>
```

> Note: [Chai](https://www.chaijs.com) is _an assertion library_. To run the actual tests, it is better to pair [Chai](https://www.chaijs.com) with a testing framework, such as [Jasmine](https://techdevblog.io/get-your-test-on-with-jasmine-the-happy-way-to-ensure-quality) or [Mocha](https://techdevblog.io/mocha-testing-the-java-in-your-script).

## Writing test cases with [Chai](https://www.chaijs.com)

Now that we have [Chai](https://www.chaijs.com) set up, let's start writing some test cases!

First, let's look at the different types of assertions [Chai](https://www.chaijs.com) provides. [Chai](https://www.chaijs.com) has three different assertion styles:

- **assert** : This is the classic, traditional way of making assertions. It's similar to the built-in `assert` module in node.js.
- **expect** : This is a more natural language-style of making assertions. It's easier to read and write, and it's the most popular way of using Chai.
- **should** : This is a BDD (Behavior Driven Development) style of making assertions. It's similar to the `expect` style, but with a slightly different syntax.

We'll be using the `expect` style in this tutorial, as it's the most widely used and the syntax your are most likely to encounter.

### Making simple assertions

Let's start by writing a simple test case to make sure a function returns the correct output.

Here's our function:

```
function add(a, b) {
  return a + b;
}

```

And here's our test case:

```
it('should add two numbers', function() {
  expect(add(2, 3)).to.equal(5);
});

```

In this test case, we're using the `expect` function to pass the output of the `add` function to [Chai](https://www.chaijs.com). Then, we're using the `to.equal` function to make an assertion that the output is equal to 5.

If the assertion is true, the test case will pass. If it's false, the test case will fail.

### Asserting the type of a variable

Sometimes, you might want to make sure that a variable is of a certain type. [Chai](https://www.chaijs.com) provides the `to.be.a` function for this.

Here's an example:

```
it('should be a string', function() {
  expect(myVariable).to.be.a('string');
});

```

In this test case, we're using the `to.be.a` function to assert that `myVariable` is a string.

### Asserting the length of an array or string

You can also use [Chai](https://www.chaijs.com) to assert the length of an array or string.

Here's an example:

```
const myArray = [0, 1, 2];

it('should have the correct length', function() {
  expect(myArray).to.have.lengthOf(3);
});

```

In this test case, we're using the `to.have.lengthOf` function to assert that `myArray` has a length of 3.

### Asserting the presence or absence of properties

You can use [Chai](https://www.chaijs.com) to assert the presence or absence of properties on an object.

Here's an example of asserting the presence of a property:

```
const myObject = { 'name': 'some name' };

it('should have the correct property', function() {
  expect(myObject).to.have.property('name');
});

```

And here's an example of asserting the absence of a property:

```
const myObject = { 'name': 'some name' };

it('should not have the incorrect property', function() {
  expect(myObject).to.not.have.property('age');
});

```

### Making complex assertions

Sometimes, you might want to make more complex assertions about your code. [Chai](https://www.chaijs.com) provides a variety of functions that allow you to do this.

For example, let's say we want to make sure that a variable is an array and contains a specific set of values. Here's how we could do that:

```
const myArray = [-2, 0, 1, 2, 3, 4, 5];

it('should have the correct values', function() {
  expect(myArray).to.be.an('array').that.includes(1, 2, 3);
});

```

In this test case, we're using the `to.be.an` function to assert that `myArray` is an array, and the `that.includes` function to assert that it contains the values 1, 2, and 3.

In these test cases, we're using the `to.have.property` and `to.not.have.property` functions to assert the presence or absence of the `name` and `age` properties on `myObject`.

### Asserting the throw of an error

Sometimes, you might want to make sure that a function throws an error under certain conditions. [Chai](https://www.chaijs.com) provides the `to.throw` function for this.

Here's an example:

```
it('should throw an error', function() {
  expect(myFunction).to.throw();
});

```

In this test case, we're using the `to.throw` function to assert that `myFunction` throws an error.

## Wrapping up

And that's it! You now know the basics of using [Chai](https://www.chaijs.com) to write test cases for your JavaScript code. With these tools in your arsenal, you'll be able to write powerful and elegant test suites in no time.

Happy testing!

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