Proposal: Safe type assertion in expression

[kgtkr]

Background

Currently, it is impossible to add type ascription only to variable declaration.
Also, casting is not safe.
I want a way to safely add type ascription in the middle of the expression.

Proposal

expression :: Type

Example

//equal: const x:[number,string]=[1,"str"];
const x=[1,"str"]::[number,string];

declare const x: {};
const a: { p: number } = x;//err
const a = x as { p: number };//ok
const a = x :: { p: number };//err

[MartinJohns]

Could you expand a bit more on why you would want this? It just seems to be more syntax that needs to be maintained for a functionality that already exists and works.

[kgtkr]

example:

const obj={
  x:"abc"::"abc"|"xyz", 
  //more props
};

//verbose
const obj:{x:"abc"|"xyz",...}={
  x:"abc", 
  //more props
};

//unsafe
const obj={
  x:"abc" as "abc"|"xyz", 
  //more props
};

I gave a simple example, but I think that it will be more useful in various situations
It can be used for upcast etc.

[krryan]

Not wild about the :: syntax, but strongly support the idea.

As a work around, our project uses function to<T>(value: T): T { return value; } so that you can write something like

const obj = {
    x: to<'abc' | 'xyz'>('abc'),
    // more props
};

Kind of annoying to have to use this function, though, plus it incurs runtime overhead for no good reason.

[kgtkr]

I noticed that :: and esnext:bind-operator became ambiguous
I have to find another syntax

https://github.com/tc39/proposal-bind-operator

[RyanCavanaugh]

Same general problem as #7481

[chinoto]

7481 was closed due to length. Continued at #47920. I figure this might help anyone who lands here get up to speed quicker.