prefer-as-const

Recommends using const assertion (as const) over explicitly specifying literal types or using type assertion.

When declaring a new variable of a primitive literal type, there are three ways:

1. adding an explicit type annotation
2. using normal type assertion (like as "foo", or <"foo">)
3. using const assertion (as const)

This lint rule suggests using const assertion because it will generally lead to a safer code. For more details about const assertion, see the official handbook.

Invalid:

let a: 2 = 2; // type annotation
let b = 2 as 2; // type assertion
let c = <2> 2; // type assertion
let d = { foo: 1 as 1 }; // type assertion

Valid:

let a = 2 as const;
let b = 2 as const;
let c = 2 as const;
let d = { foo: 1 as const };

let x = 2;
let y: string = "hello";
let z: number = someVariable;