`import type` in TypeScript

TypeScript 3.8 will support a new import type construct. While there’s already some logic for similar stuff (Flow has its own import type), TypeScript’s has a few extra restrictions.

• import type can only be used on imports with named imports or with a default import - but not both.

  import type FooDefault, { Bar, Baz } from "module"; // error!
  

• Even though import type creates no emit, an import type functionally shadows variables from the outer scope, and using these declarations in a value position that will be emitted causes an error.

  import type { RegExp } from "special-regexp-module";
  
  // Okay - RegExp won't be used at runtime here.
  declare class Foo extends RegExp { }
  
  // Okay - RegExp won't be used at runtime here.
  let r: typeof RegExp;
  
  // Error! RegExp is imported via import type and cannot be used as a value here.
  new RegExp()
  

See more at

• https://github.com/microsoft/TypeScript/pull/35200
• https://github.com/microsoft/TypeScript/pull/36092
• https://github.com/microsoft/TypeScript/issues/36138

[ExE-Boss]

I thought this was the TypeScript repository when I posted my previous comment.

[jridgewell]

Currently, we use a .type property for flow imports. It would be nice to keep them aligned.

The explorer suggests we use importKind: 'type' for type imports and importKind: 'value' for value imports.