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
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?!

@ts-check

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.

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-erroron the preceding line.

js
// @ts-check
/** @type {number} */
var x;

x = 0; // OK
x = false; // Not OK
// Type 'boolean' is not assignable to type 'number'.

// @ts-expect-error
x = false; // Not OK

## Type Checking JavaScript Files
## JSDoc Reference
The list below outlines which constructs are currently supported when using JSDoc annotations to provide type information in JavaScript files.

JS Projects Utilizing TypeScript

TypeScript with JavaScript

Providing Type Hints in JS via JSDoc

@ts-check

Creating .d.ts Files from .js files