this-based type guards

TypeScript 1.8 extends user-defined type guard functions to class and interface methods.

this is T is now valid return type annotation for methods in classes and interfaces.When used in a type narrowing position (e.g. if statement), the type of the call expression target object would be narrowed to T.

Example
  2. class FileSystemObject {
  3.     isFile(): this is File { return this instanceof File; }
  4.     isDirectory(): this is Directory { return this instanceof Directory;}
  5.     isNetworked(): this is (Networked & this) { return this.networked; }
  6.     constructor(public path: string, private networked: boolean) {}
  7. }
  9. class File extends FileSystemObject {
  10.     constructor(path: string, public content: string) { super(path, false); }
  11. }
  12. class Directory extends FileSystemObject {
  13.     children: FileSystemObject[];
  14. }
  15. interface Networked {
  16.     host: string;
  17. }
  19. let fso: FileSystemObject = new File("foo/bar.txt", "foo");
  20. if (fso.isFile()) {
  21.     fso.content; // fso is File
  22. }
  23. else if (fso.isDirectory()) {
  24.     fso.children; // fso is Directory
  25. }
  26. else if (fso.isNetworked()) {
  27.     fso.host; // fso is networked
  28. }