Up until TypeScript 2.2, type checking and error reporting were only available within .ts files. Starting with TypeScript 2.3, the compiler can now type-check and report errors for plain .js files as well:

let foo = 42; // [js] Property 'toUpperCase' does not exist on type 'number'. let upperFoo = foo.toUpperCase();

There's a new --checkJs flag which enables type checking for all .js files by default. Additionally, three new directives in the form of comments allow for more granular control over which pieces of JavaScript code should be checked:

Use // @ts-check to opt in to type checking for a single file .

to to type checking for a . Use // @ts-nocheck to opt out of type checking for a single file .

to of type checking for a . Use // @ts-ignore to opt out of type checking for a single line.

These options leave you with a blacklist approach and a whitelist approach. Note that either way, the --allowJs option should be set to true so that JavaScript files are allowed to be included in the compilation in the first place.

The Blacklist Approach

The idea behind the blacklist approach is to type-check every JavaScript file by default. This can be achieved by setting the --checkJs compiler option to true . You can blacklist specific files by adding the // @ts-nocheck comment at the top of each of these files.

I would recommend this approach if you have a smallish JavaScript code base that you want to type-check in one go. If an error is reported, you can either fix it right away, ignore the line that caused the error using // @ts-ignore , or ignore the entire file using // @ts-nocheck .

The Whitelist Approach

The idea behind the whitelist approach is to only type-check selected JavaScript files by default. This can be achieved by setting the --checkJs compiler option to false and adding the // @ts-check comment at the top of each of the selected files.

I would recommend this approach if you want to gradually introduce type checking in a large JavaScript code base. This way, you won't be overwhelmed with too many errors all at once. Whenever you're working on a file, consider adding // @ts-check and fixing potential type errors first, effectively implementing a creeping migration.

Migrating from JavaScript to TypeScript