const assertions

A const assertion is a type assertion where const is the type given. It limits the scope of whatever is cast to the most specific form the expression can take.

TypeScript documentationThe relevant pull request

Syntax

Where a normal type assertion would look something like:

const fruit = "banana" as string;

// fruit is string

A const assertion looks like:

const fruit = "banana" as const;

// fruit is "banana"

If not in TSX, you can use the alternative angle bracket assertion syntax instead:

const fruit = <const>"banana";

Caveats

These only work with literal expressions, object literals & array literals. As in, this won't work:

// error!
const millisInMinute = (1000 * 60) as const;

// no error 😌
const millisInMinute = 60000 as const;

They also don't convert an expression to be fully immutable, so this is possible:

const fruits = ["banana", "apple"];

const shelf = { fruits } as const;

// woah!
shelf.fruits.push("pomegranate");