JS Projects Utilizing TypeScript

The type system in TypeScript has different levels of strictness when working with a codebase:

  • A type-system based only on inference with JavaScript code
  • Incremental typing in JavaScript via JSDoc
  • Using // @ts-check in a JavaScript file
  • TypeScript code
  • TypeScript with strict enabled

Each step represents a move towards a safer type-system, but not every project needs that level of verification.

TypeScript with JavaScript

This is when you use an editor which uses TypeScript to provide tooling like auto-complete, jump to symbol and refactoring tools like rename. The homepage has a list of editors which have TypeScript plugins.

Providing Type Hints in JS via JSDoc

In a .js file, types can often be inferred. When types can’t be inferred, they can be specified using JSDoc syntax.

JSDoc annotations come before a declaration will be used to set the type of that declaration. For example:

/** @type {number} */ var x; x = 0; // OK x = false; // OK?!Try

You can find the full list of supported JSDoc patterns in JSDoc Supported Types.


The last line of the previous code sample would raise an error in TypeScript, but it doesn’t by default in a JS project. To enable errors in your JavaScript files add: // @ts-check to the first line in your .js files to have TypeScript raise it as an error.

// @ts-check /** @type {number} */ var x; x = 0; // OK x = false; // Not OK Type 'boolean' is not assignable to type 'number'.2322Type 'boolean' is not assignable to type 'number'.Try

If you have a lot of JavaScript files you want to add errors to then you can switch to using a jsconfig.json. You can skip checking some files by adding a // @ts-nocheck comment to files.

TypeScript may offer you errors which you disagree with, in those cases you can ignore errors on specific lines by adding // @ts-ignore or // @ts-expect-error on the preceding line.

// @ts-check /** @type {number} */ var x; x = 0; // OK // @ts-expect-error x = false; // Not OKTry

To learn more about how JavaScript is interpreted by TypeScript read How TS Type Checks JS

The TypeScript docs are an open source project. Help us improve these pages by sending a Pull Request

Contributors to this page:
OTOrta Therox  (5)

Last updated: Aug 05, 2020