declareconst

declareconst refers to the TypeScript construct declare const, an ambient declaration used to describe a global constant that exists at runtime but is defined outside the TypeScript source. It provides the compiler with the name and type of the value without emitting any JavaScript code.

Syntax and scope: The typical form is declare const name: Type;. This can appear in a declaration file (.d.ts) or inside a global augmentation. When used in a module, it still describes a global, ambient constant provided by the environment rather than a value exported by the module. The constant is treated as a read-only value for type-checking purposes.

Usage scenarios: declare const is common when integrating with environments, libraries, or build setups that expose global constants (for example, API_ENDPOINT, VERSION, or environment variables injected at runtime). It allows TypeScript code to reference these values with proper types without importing them as modules.

Semantics and behavior: Declaring a constant with declare const tells the TypeScript compiler that the identifier exists and has the specified type, but it does not generate code for the declaration. The value is considered read-only in type terms (you should not assign to it in TypeScript). At runtime, the actual value must be provided by the environment to avoid runtime errors.

Examples: declare const API_ENDPOINT: string; declare const SUPPORTS_FEATURE_X: boolean; declare const APP_CONFIG: { theme: string; locale: string };

Relation to other ambient declarations: declare const is part of a family that includes declare function, declare var, and declare class. The key distinction is that const conveys read-only, constant semantics for the declared value.