• A type-system based only on inference with JavaScript code
• Incremental typing in JavaScriptvia JSDoc
• Using// @ts-check in a JavaScript file
• TypeScript code
• TypeScript withstrict 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.Thehomepage 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 that come before a declaration will be used to set the type of that declaration. For example:

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

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

@ts-check

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.

js
// @ts-check
/**@type{number} */
varx;
 
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 ajsconfig.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.

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

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