Why does TypeScript accept value as a data type?

Question

Why does TypeScript accept value as a data type?

These scenarios below are accepting and non-acceptable declarations.

export class MyComponent{
           


        

Answer 1

Why does TypeScript accept a value as a data type?

This is extension of string literal types, this PR explains it: literal types

How does JavaScript handle these at compile time?

Its pure typescript creation, that will not affect resulting javascript.

How does it differ from readonly and constant?

Well - it will not be readonly. It will just allow one value. Check this example:

export class MyComponent
{
    readonly error = 1;
    error1: 1 = 1;

    public do()
    {
        this.error = 1; //Error. The field is readonly
        this.error1 = 1; //No error - because the field is not readonly
        this.error1 = 2; //Error. Type mismatch
    }
}