With TypeScript version 3.4, we now have the option to define literals with as const. Const assertion is useful when creating an immutable variable.

const vs as const

At first, as const may seem redundant because it is possible to declare a variable with const and make the value be unchangeable (line 1).

However, const and as const differ. With as const (line 3), the declaration is done with let, allowing the value to be changed. But, this change is done only to the literal type. Here is an example: