Add feature to re-declare variable

Search 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.

Inizia con gli esempi proposti di redeclare e la dichiarazione esistente in lib/lib.dom.d.ts, in particolare il caso URLSearchParams. Esamina come TypeScript gestisce attualmente le dichiarazioni di variabili globali e le estensioni, quindi definisci il comportamento per la sovrascrittura e l'unione delle dichiarazioni. Il lavoro sarà considerato completato quando la proposta avrà definito la semantica e saranno presenti test del compilatore corrispondenti per gli esempi.