12. Migrating JS to TS

Migrating from JavaScript to TypeScript can bring significant advantages to your project, such as improved code quality, better tooling support, and enhanced type safety. In this article, we’ll discuss the benefits of migrating to TypeScript, how to add TypeScript to existing projects, and strategies for handling dynamic types during the migration process.

Benefits of Migrating to TypeScript

1. Type Safety

TypeScript introduces static typing, allowing you to catch type-related errors at compile time rather than runtime. This results in more robust and predictable code, reducing the likelihood of runtime errors and unexpected behavior.

2. Better Tooling Support

TypeScript offers excellent tooling support through code editors like Visual Studio Code. Features such as autocompletion, type checking, and refactoring tools can significantly boost developer productivity.

3. Improved Code Maintainability

TypeScript encourages writing clean and well-documented code by enforcing type annotations and interfaces. This leads to better code readability and maintainability, making it easier for developers to work on the project.

4. Enhanced Collaboration

With TypeScript’s strong typing, it’s easier for teams to collaborate on larger projects. Type definitions act as self-documentation, making it clear how functions and modules should be used.

5. Incremental Adoption

You can gradually migrate a JavaScript project to TypeScript. This means you don’t have to rewrite the entire codebase at once. You can start by adding type annotations to specific parts of the code, gradually increasing type coverage.

Adding TypeScript to Existing Projects

Adding TypeScript to an existing JavaScript project involves a few key steps:

1. Installation

You need to install TypeScript and its compiler, tsc, as development dependencies:

npm install --save-dev typescript

2. tsconfig.json

Create a tsconfig.json file in the project root to configure TypeScript options. You can generate a basic configuration file using:

npx tsc --init

Then, adjust the configuration as needed. Ensure that TypeScript is aware of your JavaScript files by including them in the "include" section.

3. Type Definitions

To get type-checking benefits, you’ll need type definitions for libraries you use. You can install type definitions for popular libraries using @types packages:

npm install --save-dev @types/library-name

4. Conversion

Start converting your JavaScript files to TypeScript by renaming them with the .ts extension. Begin by adding type annotations to variables, functions, and method signatures.

5. Resolve Type Errors

After converting code, run tsc to identify type errors. Address each error by adding type annotations or fixing type-related issues.

6. Gradual Migration

Migrate your code incrementally. It’s acceptable to keep some parts as JavaScript while gradually converting others. This way, you can maintain the project’s functionality throughout the migration.

Handling Dynamic Types

JavaScript is dynamically typed, allowing variables to change types during runtime. TypeScript, on the other hand, promotes static typing. When migrating, you may encounter situations where dynamic types are needed. Here are strategies to handle them:

1. Use the any Type

TypeScript provides the any type, which allows you to assign values of any type to a variable. While overusing any should be avoided, it can be helpful during the migration process when dealing with unclear or dynamic types.

let dynamicValue: any = someFunctionThatReturnsDynamicValue();

2. Type Assertion

You can use type assertions to inform TypeScript of a variable’s type when you have more knowledge about it than TypeScript’s type inference system.

let dynamicValue: unknown = someFunctionThatReturnsDynamicValue();
let stringValue: string = dynamicValue as string;

3. Create Union Types

When dealing with variables that can have multiple types, consider using union types to express that flexibility explicitly.

let dynamicValue: string | number = someFunctionThatReturnsDynamicValue();

4. Unknown Type

TypeScript 3.0 introduced the unknown type, which is a safer alternative to any. You can use it when you don’t know the type of a variable, and you must perform type-checking before using it.

let dynamicValue: unknown = someFunctionThatReturnsDynamicValue();

if (typeof dynamicValue === "string") {
    // Now TypeScript knows it's a string
    let stringValue: string = dynamicValue;


Migrating from JavaScript to TypeScript offers various benefits, including type safety, improved tooling support, and enhanced code quality. By following a structured process, you can add TypeScript to your existing projects incrementally. Handling dynamic types may require using any, type assertions, union types, or the unknown type depending on the specific scenario. Migrating to TypeScript can lead to more robust and maintainable codebases, making it a worthwhile investment for your projects.