Mixing TypeScript and JavaScript

The integration of TypeScript and JavaScript within a single codebase is a common scenario, especially when transitioning gradually or working on projects with existing JavaScript components. This process involves understanding how TypeScript accommodates JavaScript code, handling interoperability challenges, and making strategic decisions for an effective coexistence.

TypeScript's --allowJs Flag

To incorporate JavaScript files into a TypeScript project, the --allowJs compiler flag must be enabled. This flag allows TypeScript to compile JavaScript code alongside TypeScript code. Consider a scenario where you have an existing JavaScript file, app.js, and a TypeScript file, app.ts, in the same project:

// tsconfig.json { "compilerOptions": { "allowJs": true, // Other options... }, // Other configurations... }

Now, TypeScript can seamlessly work with both JavaScript and TypeScript files in the same project.

Importing and Using JavaScript Files

Import JavaScript modules as usual in TypeScript files:

import * as utils from './utils.js'; const result = utils.calculateSum(5, 3);

Type Assertions and Declaration Files

When working with mixed TypeScript and JavaScript code, TypeScript might not infer types for JavaScript components automatically. Type assertions, using the as keyword, can be employed to provide type information explicitly. Additionally, creating declaration files for existing JavaScript modules allows TypeScript to understand their structure and types. Consider an example where you have a JavaScript module, math.js, and a TypeScript file, app.ts:

// math.js export function add(a, b) { return a + b; }
// app.ts import { add } from './math'; // Type assertion for JavaScript function const result: number = (add as (a: number, b: number) => number)(2, 3);

Gradual Adoption and Strategic Decisions

One of the strengths of TypeScript-JavaScript coexistence is the ability to adopt TypeScript gradually. Developers can choose specific modules or files to convert to TypeScript while leaving the rest as JavaScript. This allows teams to embrace TypeScript's benefits without the need for an immediate, full-scale migration. Strategic decisions, such as defining clear boundaries between TypeScript and JavaScript code, help maintain a cohesive and understandable codebase.

Interoperability and --checkJs Flag

The --checkJs compiler option enables TypeScript to perform type checking on JavaScript files. This ensures that TypeScript provides feedback and type safety even in JavaScript components. This can be valuable during the transition phase when both TypeScript and JavaScript files coexist.

// tsconfig.json { "compilerOptions": { "allowJs": true, "checkJs": true, // Other options... }, // Other configurations... }

Using TypeScript Types in JavaScript

While JavaScript doesn't natively understand types, TypeScript types can be used for tooling and IDE support.

// utils.js export function calculateSum(x: number, y: number): number { return x + y; }
// app.ts import * as utils from './utils.js'; const result: number = utils.calculateSum(5, 3); // Type is inferred

Potential Limitations

  1. Type Checking in JavaScript: TypeScript's type checking is limited in JavaScript files.
  2. Use checkJs option for some type checking, but it's not comprehensive.

Best Practices

  1. Gradually convert JavaScript to TypeScript for full type safety benefits.
  2. Use type annotations in JavaScript files for better tooling support.
  3. Consider using JSDoc comments for basic type hints in JavaScript.

Example with Mixed File Types

// app.ts (TypeScript) import * as utils from './utils.js'; // Importing a JavaScript module const myNumber: number = 10; const result = utils.multiplyByTwo(myNumber); // Using a JavaScript function console.log(result); // Output: 20
// utils.js (JavaScript) export function multiplyByTwo(num) { return num * 2; }


Mixing TypeScript and JavaScript involves seamlessly integrating both languages within a single codebase, allowing for a gradual transition or collaboration between existing JavaScript components and newly written TypeScript code. Utilizing TypeScript's --allowJs and --checkJs compiler flags, along with strategic use of type assertions and declaration files, ensures smooth interoperability and develops a cohesive development environment.