Ambient Declarations
Using declare
The declare keyword describes an existing value, function, class, or other declaration that TypeScript should assume exists at runtime without TypeScript itself generating any code for it.
Example: Using declare
declare const VERSION: string;
// Assume VERSION is injected by the build tool at runtime.
console.log("Would use VERSION here in a real ambient setup");Ambient Interfaces
Ambient interfaces can describe the shape of existing objects, such as ones provided by a browser API or a third-party script, without needing to provide any runtime implementation.
Example: Ambient Interfaces
declare interface Window {
myGlobalFlag: boolean;
}
// Describes a value assumed to exist on window at runtime.
console.log("Ambient interface declared for Window.myGlobalFlag");Ambient Functions
Ambient function declarations describe functions implemented outside the current TypeScript file — often in plain JavaScript, a <script> tag, or a native environment — so TypeScript can still type-check calls to them.
Example: Ambient Functions
declare function trackEvent(name: string): void;
// trackEvent is assumed to be implemented elsewhere (e.g. a <script> tag).
console.log("Ambient function trackEvent declared");Ambient Modules
An ambient module declaration describes the types exported by a JavaScript module that doesn't ship its own TypeScript types, effectively writing type information for a library from the outside.
Example: Ambient Modules
declare module "legacy-lib" {
export function doSomething(): string;
}
// Provides types for a JS module with no types of its own.
console.log("Ambient module declared for legacy-lib");Declaration Files
Ambient declarations are commonly stored in .d.ts declaration files, which provide type information to the rest of a project without containing any executable code themselves.
Example: Declaration Files
// types.d.ts (declaration file, no runtime code)
// declare function legacyAdd(a: number, b: number): number;
console.log("Declaration files contain only type information, no executable code");Chapter Quiz — Complete all 7 topics to unlock
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