A Practical Guide to Migrating JavaScript to TypeScript

TypeScript doesn’t demand an all-or-nothing rewrite. One of its biggest strengths is how smoothly it integrates into existing JavaScript codebases without derailing momentum.

In this lesson, we’ll look at proven strategies for progressively adopting TypeScript. From enabling compiler flags to annotating with JSDoc and wrangling third-party types, we’ll see how to turn real-world JavaScript into robust TypeScript, step by step.

Why migrate?

Most teams don’t start from scratch. They inherit large JavaScript projects—some well-structured, some… not. Migrating such code to TypeScript gives us:

• Early bug detection without writing tests.

• Safer refactors and better tooling.

• Confidence when onboarding new contributors.

And most importantly? We can do it gradually. That’s the point. Let’s break down how.

Step 1: Enable TypeScript for .js files

The first step in any migration plan is to let the TypeScript compiler see your JavaScript.

Note: Click “Run” to open the terminal, then type tsc and hit “Enter” to run the type checker.

Explanation:

• Line 3: allowJs: true allows TypeScript to include .js files in the compilation context.

• Line 4: checkJs: true turns on type checking for those .js files.

• Line 5: noEmit: true tells the compiler not to output any .js or .d.ts files—it’s purely ...