TypeScript provides developers many advantages over JavaScript: you can identify and enforce data types, better communicate the purpose of code, and streamline your development process with enhanced developer tools.

This article explains the difference between TypeScript and JavaScript, how to take advantage of the new features, and how to migrate your projects.

What is JavaScript?

JavaScript is a high-level interpreted programming language that's been around since 1995 and is widely used in web development.

It was originally developed for adding interactivity and functionality to HTML web pages. It can manipulate the Document Object Model (DOM), making it possible to create dynamic and responsive user interfaces that run in the web browser.

Although JavaScript is best known for being a client-side language, developers' widespread familiarity with it led to it being used server-side using Node.js, and to develop mobile and desktop apps using frameworks like Ionic and Electron.

What is TypeScript?

TypeScript is a programming language that's a superset of JavaScript — building on top of it to add static typing so you can catch errors during development instead of at runtime.

It was developed by Microsoft to address the difficulties of maintaining large applications written in JavaScript, and has been expanded to add a number of modern developer-friendly features.

In addition to static typing, TypeScript also adds features like interfaces, generics, type inference, access modifiers, and other constructs not present in standard JavaScript.

TypeScript compiles to plain JavaScript, so anything that works in JavaScript works in TypeScript, including third-party JavaScript libraries and any code you've already written in JavaScript.

What's the difference between TypeScript and JavaScript?

Both TypeScript and JavaScript are widely used for web development. As they are so closely related, they're often confused, but the key differences between TypeScript and JavaScript are as follows:

What is static typing?

Static typing is a programming language feature that lets you explicitly specify the data types of variables, function parameters, and return values in advance, so they are known at compile time. For TypeScript, this means the code is checked for type-related errors when it's compiled to JavaScript, so type-related errors are caught before they reach users.

By contrast, JavaScript has dynamic typing. This is where data types are determined and checked at runtime, meaning that if there is a type-related error (like trying to multiply a string by an integer), it won't be encountered until someone finds the bug by running the code — which could be one of your users in production.

Static typing in TypeScript ensures that variables are used in ways consistent with their defined types, preventing certain types of errors that can occur due to incorrect type usage.

Example of static typing in TypeScript

An example of a type error is shown in the below JavaScript snippet. This code assigns a string to the age variable, even though it was originally supposed to be a number. This leads to a runtime error when the code later tries to use age in a numeric context.

let age = 25;
age = "twenty six";
console.log(age * 2); // runtime error: "NaN"

The above JavaScript causes a NaN error in the browser.

Using a string in a numerical context results in NaN (Not a Number), a special value that indicates an operation has failed due to a number conversion.

With TypeScript, adding a static number type to the age variable causes the compiler to immediately raise the issue:

let age: number = 25;
age = "twenty six"; // compiler error: "Type 'string' is not assignable to type 'number'."

The above TypeScript causes an error earlier — before runtime.

Benefits of static typing

• Early error detection — Type-related errors are caught at compile time, helping developers identify and fix issues before the code is executed. Many modern code editors (e.g., Visual Studio Code) support the TypeScript compiler running automatically in the background, meaning errors are caught as you write code.
• Improved code quality — Type annotations make your code self-documenting and easier to understand.
• Enhanced tooling — IDEs and code editors can provide better auto-completion, code navigation, and refactoring suggestions based on type information.
• Readability and maintainability — Explicit types make code more readable and understandable, especially in larger projects.

TypeScript vs. JavaScript: use cases

When to use JavaScript

• When learning to code for the first time — Beginner developers may prefer to experiment with JavaScript's flexible code without being overwhelmed with extra information.
• For ad-hoc scripts or automation tasks — When you're automating tasks or writing quick, throwaway scripts, JavaScript is quicker to set up and get started with.
• When working on legacy codebases — If you're working on a very old codebase already written in JavaScript, it may not be worth converting it all to TypeScript, which could require significant rewrites or break existing functionality.

When to use TypeScript (hint: whenever you can!)

You should use TypeScript whenever you can. The advantages it provides are numerous, and in any application of any scale, it gives you a better developer experience that results in more reliable code. This is particularly important for complex projects that involve collaboration between teams.

Is TypeScript better than JavaScript?

TypeScript vs. JavaScript has a clear winner for large collaborative projects. Here's why TypeScript can be a game-changer:

• Reduced errors — Static typing helps catch type-related errors at compile time, preventing many common mistakes from reaching runtime.
• Improved tooling and IDE support — TypeScript provides robust tooling, including code completion, navigation, and refactoring suggestions.
• Enhanced collaboration — Type annotations provide clear interfaces for functions and modules, making it easier for team members to understand how different parts of the codebase interact.
• Safer refactoring — TypeScript's type system makes refactoring safer by alerting developers to potential issues when they change code.
• Code quality and consistency — TypeScript encourages a higher level of code quality and consistency by enforcing type standards.
• Code maintainability — Static typing makes it easier to understand the data types of variables, function parameters, and return values, resulting in more self-documenting code.

You don't need to take an all-or-nothing approach. You can adopt TypeScript incrementally — start by adding types to new code or the most error-prone parts of your codebase.

How to migrate your existing JavaScript project to TypeScript

Ensure you have the latest versions of Node.js and npm installed, then follow these steps:

1. Install TypeScript

npm install typescript --save-dev

2. Create tsconfig.json

npx tsc --init

This creates a basic TypeScript configuration.

3. Enable JavaScript support

To allow TypeScript to compile .js files during migration, add "allowJs": true to the compilerOptions object in tsconfig.json:

{
  "compilerOptions": {
    "allowJs": true
  }
}

4. Convert your JavaScript files to TypeScript

Rename *.js files to *.ts (or *.jsx to *.tsx). Then run the compiler:

npx tsc path/to/your/file.ts

You can also convert all files at once and run npx tsc to compile them all.

Avoid renaming auto-generated files (e.g., those in .next/) or files from external libraries in node_modules/.

5. Add type annotations gradually

Start small by adding types for function parameters and return values:

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

Then progressively add interfaces, type aliases, and more advanced TypeScript features.

For third-party libraries, you can find type definitions in the DefinitelyTyped repository.

6. Run tests

After migrating your files, run your test suites to check that everything still works and catch any breaking changes introduced during the migration.