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.

[dguttman-jacada]

@DanielRosenwasser my answer would be:

1. depending on context of execution flow . Even today, typescript knows that for:

   function foo(input: string | number): string {
     if (typeof input === "string") {
       return input; // here input is string
     } else {
       return number.toString(); // here input is number
     }
   } 
   

   so in your example, hello would end up a string (if the code is processed sequentially)
2. It makes sense for the 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:

   app.get("/", (req, res, next) => {
     redeclare req: typeof req & { myProp: string };
     req.myProp = Guid.generate();
     asyncOp((res) => {
       res.send(res.req.myProp);
     });
   });
   
   req.myProp = ""; // error - req does not have a 'myProp' property
   

3. in JavaScript it is also possible to do:

   // file1.js
   var hello = 123; // essentially declare it as a number, would work with declare var hello: string | number;
   
   // file2.js
   var hello = "foo"; // essentially declare it as a string, would work with declare var hello: string | number;
   
   // file3.js
   var hello = false; // essentially declare it as a boolean, would *not* work with the above and would need to be redeclared - in most cases: redeclare var hello: typeof hello | boolean;
   

   currently there is no way to describe this with TypeScript

[sdrsdr]

redeclare can have other benefits:

I have a “header” type of interface:

export interface api_msg_t {
	a:string;
	devid:string;
}

function is_api_msg(m:any) :m is api_msg_t {
	return (
		m && typeof(m)=="object" && typeof(m.a)=='string' && typeof(m.devid)=='string'
	)
}

I want to have extended interface:

interface api_node_online_t extends api_msg_t {
	a:"onl";
	nid:number;
	state:node_state;
}
function is_api_node_online(m:api_msg_t) : m is api_node_online_t {
	return (m.a=='onl' && typeof(m.nid)=="number" && is_node_state(m.state));
}

if there is no inex type in api_msg_t the typeguard is_api_node_online fails to transpile as m.nid does not exist in api_msg_t. But adding index in the api_msg_t propagates it to api_node_online_t that is undesirable. if we can do

function is_api_node_online(m:api_msg_t) : m is api_node_online_t {
	redaclare m as api_msg_t & {[index:string]:unknonw};
	return (m.a=='onl' && typeof(m.nid)=="number" && is_node_state(m.state));
}

It will allow for elegant and typesafe solution.

We can easily mimic this behavior with

interface api_msg_indexed_t extends api_msg_t {
	[index:string]:nknown;
}

function is_api_msg_indexed(m:api_msg_t):m is api_msg_indexed_t:{
	return true;
}

function is_api_node_online(m:api_msg_t) : m is api_node_online_t {
	if (!api_msg_indexed(m)) return;
	return (m.a=='onl' && typeof(m.nid)=="number" && is_node_state(m.state));
}

but this results in unnecessary JS code emitted and ran