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TypeScript FAQs

Should TypeScript should be installed globally or locally?​

Make sure that you have installed TypeScript locally i.e. by using npm install typescript, not npm install -g typescript, or by using yarn add typescript, not yarn global add typescript. See #2041 for more information.

Why don't I see TypeScript errors in my ESLint output?​

TypeScript's compiler (or whatever your build chain may be) is specifically designed and built to validate the correctness of your codebase. Our tooling does not reproduce the errors that TypeScript provides, because doing so would slow down the lint run [1], and duplicate the errors that TypeScript already outputs for you.

Instead, our tooling exists to augment TypeScript's built in checks with lint rules that consume the type information in new ways beyond just verifying the runtime correctness of your code.

[1] - TypeScript computes type information lazily, so us asking for the errors it would produce from the compiler would take an additional ~100ms per file. This doesn't sound like a lot, but depending on the size of your codebase, it can easily add up to between several seconds to several minutes to a lint run.

How can I specify a TypeScript version / parserOptions.typescriptLocation?​

You can't, and you don't want to.

You should use the same version of TypeScript for linting as the rest of your project. TypeScript versions often have slight differences in edge cases that can cause contradictory information between typescript-eslint rules and editor information. For example:

  • @typescript-eslint/strict-boolean-expressions might be operating with TypeScript version X and think a variable is string[] | undefined
  • TypeScript itself might be on version X+1-beta and think the variable is string[]

See this issue comment for more details.

Why aren't // @ts-expect-error or // @ts-ignore comments affecting lint results?​

// @ts-expect-error and // @ts-ignore comment directives are a feature of TypeScript. They only impact TypeScript's type checking. TypeScript (a type checker) is a separate tool from ESLint (a linter).

Similar, ESLint configuration comments like /* eslint ... */ only impact ESLint. They don't change anything with TypeScript's type checking.

tip

You can use ESLint to enforce good uses of both ESLint and TypeScript comment directives: