Extending outdated TypeScript package declarations

Published November 2, 2023・Last updated June 2, 2025・1 min read

Extending package declarations locally is one of the options for outdated package typings.

Create a declaration file .d.ts (e.g., handlebars.d.ts), and put it inside the src directories.

Find the exact name of the package namespace inside the node_modules types file (e.g. handlebars/types/index.d.ts).

Extend the found namespace with your needed properties, like classes, functions, etc.

// handlebars.d.ts
declare namespace Handlebars {
  export class JavaScriptCompiler {
    public nameLookup(
      parent: string,
      name: string,
      type: string
    ): string | string[];
  }
  export function doSomething(name: string): void;
  // ...
}