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:

  1. /** @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.

@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.

  1. // @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.

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

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