Variable merging

declare var test: (s: string) => void;

declare var test: {
    (s: string) => void;
    tests: any[];
}

At the moment TS compiler shows error: Subsequent variable declarations must have the same type.

[andy-ms]

It might be useful to support this in the particular case of a property of type any. Often DefinitelyTyped declarations will declare something to be of type any when a particular user will know a better type.

// DefinitelyTyped
interface I { foo: any; }

// My code
interface I { foo: { x: number }; }

Currently we give an error because the types for foo conflict. We might just throw out any declarations typed as any when getting the type.

[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