Is it acceptable to include both of the following in a definition file:
export declare type Abc = string;
export type Bcd = string;
Does the declare keyword serve any meaningful purpose in this context?
Is it acceptable to include both of the following in a definition file:
export declare type Abc = string;
export type Bcd = string;
Does the declare keyword serve any meaningful purpose in this context?
Indeed, the declare keyword serves a crucial purpose when indicating the existence of a variable or constant during runtime.
For instance, consider a scenario where you wish to utilize an external library named someExternalLib, which is not available on npm and needs to be manually included via a script tag. You are aware that this library will be globally accessible as someExternalLib, containing functions like fun1 and fun2. However, Typescript lacks this knowledge, hence the requirement to declare the global variable someExternalLib explicitly:
declare const someExternalLib: { fun1: () => number, fun2: () => number }
This declaration is typically essential in definition files for specifying variables, constants, classes, and functions. On the other hand, it is unnecessary for types and interfaces.
declare in TypeScript is similar to how forward declarations work in C++.
Forward declarations inform the compiler that a certain entity:
This declaration can be for anything, such as a type, object, function signature, enum, etc., without providing the actual implementation.
An example of a forward declaration in C++ could be:
int add(int x, int y);
This specifies to the C++ compiler that there exists a function taking two integer arguments and returning an integer, but the implementation is elsewhere and not seen in this declaration.
In TypeScript, the concept is similar but the implementation differs.
A declaration in TypeScript conveys to the compiler how a certain entity is structured, what types and properties it has, without providing the actual definition: you can declare a function signature or namespace with its components, but no concrete implementation (no function bodies!).
One important point about TypeScript's declare statement is that:
declare type MyObject = {
doSomething: () => void,
// ...
}
and
type MyObject = {
doSomething: () => void,
// ...
}
Although they may appear identical and behave similarly, the first version only helps in static type checking while generating type definition files (d.ts) for external libraries; the second version is necessary for proper generation unless manual definition file creation is opted for.
Think of declare as a way to describe externally sourced code to the TypeScript compiler, such as untyped JavaScript libraries or imported modules without type definitions.
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