After reading about Typescript, I learned that the 'declare' keyword is similar to the 'extern' keyword in C. It declares a variable that is defined elsewhere, possibly in the browser:
declare var Request: {
prototype: Request;
new(input: RequestInfo | URL, init?: RequestInit): Request;
};
I'm a bit confused about this part:
prototype: Request
It seems redundant to declare Request as Request.
In the example you provided, the use of var is part of a pair with interface that TypeScript standard libraries utilize to define built-in stuff like classes in TypeScript.
There are two instances of Request in your example. One represents the actual value - a var expected to exist during runtime. The other depicts the type - an erased interface that will not exist at runtime when compiled into JavaScript. These distinctions can be identified by their different syntactic contexts; for instance, if you encounter declare var x: Y, where x is the value name and Y is the type name.
This kind of declaration is common in TypeScript standard libraries as it mimics class declarations. When you declare something like:
declare class Foo {
a: string;
b: number;
constructor(a: string, b: number);
}
Both a value and a type named Foo come into play. The value represents the Foo constructor, allowing you to create instances such as new Foo("abc", 123). On the other hand, the type signifies the type of Foo instances, akin to an interface like {a: string, b: number}. Having them share the same name enables developers to write code like
const foo: Foo = new Foo("abc", 123)The standard libraries of TypeScript usually avoid using class due to backward compatibility and edge case reasons. Instead they employ a var/interface paired declaration approach:
interface Foo {
a: string;
b: number;
}
declare var Foo: {
prototype: Foo;
new(a: string, b: number): Foo;
}
This method achieves similar functionality as declaring a class explicitly but brings the type and value separately into scope. The value Foo serves as a variable with a construct signature while the interface Foo represents the instance type.
You can perform all operations with the var/interface pair just like with class. The following code behaves identically:
const foo = new Foo("abc", 123);
foo.a.toUpperCase();
foo.b.toFixed();
By examining the TypeScript library definitions for Request, you'll notice an associated interface. This could be restructured as follows:
declare class Request {
new(input: RequestInfo | URL, init?: RequestInit): Request;
readonly body: ReadableStream<Uint8Array> | null;
// More properties...
clone(): Request;
}
and would function similarly.
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