Add feature to re-declare variable
See original GitHub issueSearch Terms
redeclare variable, re-declare variable, augment variable, merge variable declaration, declaration override
Suggestion
define a redeclare
keyword which allows re-declaration of a typed variable.
e.g.
redeclare var foo: {
prop: string
};
Use Cases
When an external library declares a variable, e.g.
declare var x: {
prop: string;
};
The redeclare
keyword can be used somewhere else to override the declaration:
redeclare var x: number;
or augment it:
redeclare var x: typeof x & {
augmentedProp: number;
}
This would allow to type the pure JavaScript case of:
// lib.js
var x = {
prop: "foo"
};
// app.js
x.augmentedProp = 10;
console.log(x.prop, x.augmentedProp);
Examples
declare var URLSearchParams: { prototype: URLSearchParams; new(init?: string[][] | Record<string, string> | string | URLSearchParams): URLSearchParams; toString(): string; };
defined in
lib.dom.d.ts
to extend the variable with additional static methods to URLSearchParams
is not possible currently (see here).
The feature can be used in the following way:
redeclare var URLSearchParams: typeof URLSearchParams & {
fromObject(obj: object): URLSearchParams;
};
if (!URLSearchParams.fromObject) {
URLSearchParams.fromObject = function(obj: object): URLSearchParams { /* implementation */ }
}
Checklist
My suggestion meets these guidelines:
- This wouldn’t be a breaking change in existing TypeScript/JavaScript code
- This wouldn’t change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
- This feature would agree with the rest of TypeScript’s Design Goals.
Issue Analytics
- State:
- Created 3 years ago
- Reactions:2
- Comments:5 (2 by maintainers)
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@DanielRosenwasser my answer would be:
hello
would end up astring
(if the code is processed sequentially)redeclare
to be scoped, so if the variable was redeclared within a function body (or any other relevant scope), the redeclaration would be valid for the scope of the function body. e.g for an express app:redeclare
can have other benefits:I have a “header” type of interface:
I want to have extended interface:
if there is no inex type in
api_msg_t
the typeguardis_api_node_online
fails to transpile asm.nid
does not exist inapi_msg_t
. But adding index in theapi_msg_t
propagates it toapi_node_online_t
that is undesirable. if we can doIt will allow for elegant and typesafe solution.
We can easily mimic this behavior with
but this results in unnecessary JS code emitted and ran