TypeScript to Check JavaScript
Tags: javascript, typescript, tutorial, node
Published: 2024-01-18 13:31:50

TypeScript is a programming language that adds static types to JavaScript, allowing developers to detect and fix errors at compile time. But did you know that it's also possible to use TypeScript to check JavaScript files? In this post, I'll show you how to do that and what the advantages of this approach are.

## Why Use TypeScript to Check JavaScript?

JavaScript is a dynamic and flexible language, but that can also bring some problems. For instance, if you try to access a property that doesn't exist on an object or pass an argument of the wrong type to a function, you may get unexpected results or hard-to-debug errors. Additionally, JavaScript lacks native support for documentation tools, refactoring, and autocompletion, which can hinder productivity and code quality.

TypeScript addresses these issues by adding static types to JavaScript. This means you can specify the type of each variable, parameter, return, and property in your code. This allows the TypeScript compiler to check if your code is correct and consistent, generating type information that can be used by code editors and other tools to enhance the development experience.

But if TypeScript is so good, why not use it directly instead of JavaScript? Well, there are some possible reasons:

- You have an existing project in JavaScript and don't want or can't migrate the entire code to TypeScript.
- You want to use some JavaScript libraries or features that don't have defined types or are not compatible with TypeScript.
- You prefer the syntax and semantics of JavaScript and don't want to learn or use a new language.

In these cases, you can use TypeScript to check JavaScript files without changing your language or code. You just need to enable some settings in your **`tsconfig.json`** file and add some special comments in your JavaScript code to indicate variable types. This way, you can enjoy the benefits of TypeScript without losing the flexibility of JavaScript.

## How to Use TypeScript to Check JavaScript?

To use TypeScript to check JavaScript files, follow these steps:

1. Install TypeScript globally or locally in your project using the command **`npm install -g typescript`** or **`npm install --save-dev typescript`**.
2. Create a **`tsconfig.json`** file at the root of your project with the following content:

```json
{
  "compilerOptions": {
    "allowJs": true, // Allows JavaScript files
    "checkJs": true, // Enables checking in JavaScript files
		"noEmit": true   // Prevents compiler from emitting files
},
  "include": ["src"]
}
```

These options tell the TypeScript compiler to allow JavaScript files (**`allowJs`**), check them (**`checkJs`**), and the option (**`noEmit`**) ensures that TypeScript only checks files without generating new compiled files. The **`include`** option specifies which files should be included in the check. You can adjust these options according to your needs.

3. Add special comments in your JavaScript code to indicate variable types using JSDoc syntax. For example:

```jsx
/**
 * Adds two numbers.
 * @param {number} a The first number.
 * @param {number} b The second number.
 * @returns {number} The sum of the two numbers.
 */
function add(a, b) {
  return a + b;
}

// The TypeScript compiler will warn if you pass arguments of the wrong types
add("1", "2"); // Error: Argument of type 'string' is not assignable to parameter of type 'number'.
```

You can find more information on how to use JSDoc to document and type your JavaScript code in the [JSDoc documentation](https://jsdoc.app/).

4. Run the **`tsc`** command to compile your project and check for type errors. You can also use a code editor that supports TypeScript, such as Visual Studio Code, to see real-time errors.

## What Are the Advantages of Using TypeScript to Check JavaScript?

By using TypeScript to check JavaScript files, you can gain the following advantages:

- Increase the quality and reliability of your code by avoiding common errors and inconsistencies caused by dynamic types.
- Enhance code readability and maintainability by documenting variable types and facilitating refactoring.
- Improve productivity and the development experience by leveraging autocomplete, navigation, renaming, and other tools that use type information generated by TypeScript.
- Maintain compatibility and flexibility in your code, allowing you to use any JavaScript feature or library without changing your language or code.

## Conclusion

In this post, you've learned how to use TypeScript to check JavaScript files, why this can be useful, and what the advantages of this approach are. I hope you enjoyed it and that it helps you improve your JavaScript projects. If you have any questions or suggestions, feel free to leave a comment below.

Thanks for reading! 😊