Object.defineProperty declarations in JavaScript

When writing in JavaScript files (using allowJs), TypeScript now recognizes declarations that use Object.defineProperty.This means you’ll get better completions, and stronger type-checking when enabling type-checking in JavaScript files (by turning on the checkJs option or adding a // @ts-check comment to the top of your file).

  1. // @ts-check
  3. let obj = {};
  4. Object.defineProperty(obj, "x", { value: "hello", writable: false });
  6. obj.x.toLowercase();
  7. //    ~~~~~~~~~~~
  8. //    error:
  9. //     Property 'toLowercase' does not exist on type 'string'.
  10. //     Did you mean 'toLowerCase'?
  12. obj.x = "world";
  13. //  ~
  14. //  error:
  15. //   Cannot assign to 'x' because it is a read-only property.