It is recommended that type assertions work on the variables themselves

[zhmushan]

TypeScript Version: 2.4.0

interface I {
    data: string
}
let a: any = {data: 'string'}
a as I
a.da  // Now it should be wrong
a.data  // There should be smart tips here

We want the type assertion to work on the variable itself in the code block

[jcalz]

Related to #17760?

What I do:

Helper function:

function assert<T>(x: any): x is T {
  return true;
}

Then:

interface I {
    data: string
}
let a: any = {data: 'string'}
if (!assert<I>(a)) throw null; // like "a as I"
a.da  // error
a.data  // smart tips galore

[zhmushan]

@jcalz This method is valid
But converted to Javascript,some become redundant

function assert(x) {
    return true;
}
var a = { data: 'string' };
if (!assert(a))
    throw null; // like "a as I"
a.da; // error
a.data; // smart tips galore

if expression will always be correct, it becomes an extra code with assert()
I think your method is similar:

interface I {
    data: string
}
let a: any = {data: 'string'}
let b: I = a

Both of these methods will generate extra Javascript code

Is it possible to have a method that only plays the role of type assertions in the Typescript, without generating extra JavaScript code

[RyanCavanaugh]

Duplicate of #10421