Using TypeScript Type Guards

I've been appreciating a TypeScript pattern called type-guards.

Type-guards provide type-safety by performing a runtime check to ensure a variable is within the typed scope.

/**
 * Type-Guard: Asserts that the document is a S3 file
 */
export const isS3Document = (document: Document): document is S3Document =>
  "url" in document;

You could use the type-guard as such:

const onlyS3Files = (files: Document[]): S3Document[] =>
  files.filter(isS3Document);

To create a type-guard, you must:

• Return a type predicate (e.g. variable is S3Document)
• Contain a logical statement to determine the type of the given variable

With this in view, you can also create a generic type-guard that can be used to assert the type safety of any variable:

export const isType = <T>(variable: any, property: keyof T): variable is T =>
  (variable as T)[property] !== undefined;

const onlyS3Files = (files: Document[]): S3Document[] =>
  files.filter((document) => isType<S3Document>(document, "url"));