TypeScript `declare` Keyword

The declare keyword is used in TypeScript to create ambient declarations. It tells the compiler: "Assume this variable, function, class, or module exists at runtime, even if you can't see its implementation right now."

This is essential when using third-party JavaScript libraries that don't have type definitions, or when working with global variables injected by the environment (like the browser or a build tool).

1. Key Characteristic: No Output

Code marked with declare does not emit any JavaScript output. It is purely for the type checker during development.

declare const MY_GLOBAL_API_KEY: string;

console.log(MY_GLOBAL_API_KEY);

Compiled JS:

console.log(MY_GLOBAL_API_KEY);

(Notice the declare line is gone).

2. Declaring Globals

If a script loaded before yours defines a global variable, you need to tell TypeScript about it so it doesn't throw a ReferenceError during compilation.

Variables

declare var jQuery: (selector: string) => any;

jQuery('#app').hide();

Functions

declare function ga(command: string, ...args: any[]): void;

ga('send', 'pageview');

Classes

declare class Camera {
    capture(): void;
}

let c = new Camera();

3. `declare module`

This is used to define types for modules that don't have their own type definitions. This is often done in .d.ts files.

declare module "my-untyped-library" {
    export function doSomething(): void;
}

4. `declare global`

If you need to add properties to the global scope (like window or globalThis) from within a module (a file with imports/exports), you must wrap the declarations in declare global.

// analytics.ts
export {}; // Make this a module

declare global {
    interface Window {
        myAnalyticsData: any;
    }
}

window.myAnalyticsData = { id: 123 };

5. `declare` in Classes

Inside a class, declare is used to tell TypeScript that a property exists, but it shouldn't emit code to initialize it. This is common when using decorators or libraries that handle initialization behind the scenes (like Babel).

class Animal {
    // Tells TS: "This field exists, don't worry about initialization"
    declare name: string; 
}

programming/javascript/typescript/typescript programming/javascript/typescript/declaration-files

Created: March 17, 2026 • Last Modified: January 30, 2026

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TypeScript declare Keyword