Test-driven development (TDD) is a software development approach in which tests are written for a piece of code before the code is written. The tests describe the desired behavior of the code, and the code is written to make the tests pass. This approach helps ensure that the code meets the requirements and works as intended.

In this post, we will explore how to do TDD in TypeScript, a typed superset of JavaScript that can be used to build large scale applications. We will cover the following topics:

  1. Setting up a TypeScript project for TDD
  2. Writing tests in TypeScript
  3. Running tests in TypeScript
  4. Debugging tests in TypeScript
  5. Best practices for TDD in TypeScript

Let’s get started!

Setting up a TypeScript project for TDD

To get started with TDD in TypeScript, you will need to set up a TypeScript project and install the necessary dependencies.

First, create a new directory for your project and navigate to it in your terminal. Then, run the following command to initialize a new npm project:

npm init -y

Next, install the TypeScript compiler and a testing library. There are many testing libraries available for TypeScript, but we will be using Jest in this tutorial. Run the following command to install these dependencies:

npm install -D typescript jest @types/jest

Now, create a tsconfig.json file in the root of your project. This file will contain the configuration options for the TypeScript compiler. Add the following configuration to the file:

{
  "compilerOptions": {
    "outDir": "./dist",
    "sourceMap": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "module": "commonjs",
    "target": "es6"
  },
  "include": ["src/**/*"]
}

Finally, create a src directory in your project and add an empty index.ts file. This will be the entry point for your TypeScript code.

Your project is now set up and ready for TDD!

Writing tests in TypeScript

To write tests in TypeScript, you will need to understand the syntax of the testing library you are using. In this tutorial, we will be using Jest.

Jest tests are written using the describe and it functions. The describe function is used to group related tests together, and the it function is used to define a single test.

Here is an example of a simple test in TypeScript:

import { add } from './math';

describe('math', () => {
  it('should add two numbers', () => {
    const result = add(1, 2);
    expect(result).toEqual(3);
  });
});

In this example, we are testing the add function from the math module. The test uses the expect function from Jest to assert that the result of the add function is equal to 3.

You can write as many tests as you like for a given piece of code. It is a good practice to test for both expected and unexpected input, to ensure that the code is robust and handles all cases properly.

Running tests in TypeScript

To run your tests in TypeScript, you will need to use the testing library’s command-line interface (CLI). In the case of Jest, you can run the tests by using the jest command in your terminal.

To run all tests in your project, simply navigate to the root of your project in your terminal and run the following command:

jest

Jest will search for test files in your project and run any tests it finds. You can also specify a particular test file or test suite to run by providing the file or suite name as an argument to the jest command:

jest path/to/test/file.ts
jest --testNamePattern='math'

By default, Jest will run tests in watch mode, which means that it will continuously watch for changes to your code and re-run the tests when a change is detected. You can disable watch mode by using the –no-watch flag:

jest path/to/test/file.ts
jest --testNamePattern='math'

By default, Jest will run tests in watch mode, which means that it will continuously watch for changes to your code and re-run the tests when a change is detected. You can disable watch mode by using the –no-watch flag:

jest --no-watch

Debugging tests in TypeScript

Debugging tests in TypeScript can be done using a debugger in your code editor or by using the debugger statement in your code.

To use a debugger in your code editor, you will need to set a breakpoint in your code. This can usually be done by clicking in the gutter of the code editor or by using a keyboard shortcut. Then, when you run your tests in debug mode, the debugger will pause execution at the breakpoint, allowing you to inspect the state of your code and step through the execution.

To use the debugger statement, simply add the debugger keyword to your code wherever you want to pause execution:

import { add } from './math';

describe('math', () => {
  it('should add two numbers', () => {
    debugger;
    const result = add(1, 2);
    expect(result).toEqual(3);
  });
});

When you run your tests in debug mode, the debugger will pause execution at the debugger statement, allowing you to inspect the state of your code and step through the execution.

Best practices for TDD in TypeScript

Here are some best practices to follow when doing TDD in TypeScript:

  • Write tests first: Following the TDD approach, always write your tests before writing the code. This will help ensure that you have a clear understanding of the requirements and can focus on writing code that meets them.

  • Test small, focused units: It’s important to test small, focused units of code rather than testing the entire application at once. This will make it easier to identify issues and fix them.

  • Test both expected and unexpected input: Make sure to test both expected and unexpected input to ensure that your code is robust and handles all cases properly.

  • Use mocks and stubs: When testing code that depends on external services or APIs, it can be helpful to use mocks or stubs to isolate the code being tested. This will make it easier to test and debug your code.

By following these best practices, you can ensure that your TypeScript code is correct, well-tested, and easy to maintain.