til/typescript/type-narrowing-with-const-vs-let-strings.md

Type Narrowing With Const VS Let Strings

TypeScript's typeof operator can be used to capture the type of a variable.

For instance, we can use it with a string variable like so:

let status = 'active';

type Status = typeof status;
//=> type Status = string;

The result is a type Status defined as a string.

What if we were to do the same with a string variable defined with const (instead of let)?

const status = 'active';

type Status = typeof status;
//=> type Status = 'active';

We get a different result. A much more specific result. The Status type definition in this case isn't just any string. TypeScript knows specifically that it is the string 'active'.

This is Type Narrowing at work. With our let string, which can be redefined at anytime, the most TypeScript can tell us about it is that it is a string. Our const string, on the other hand, is frozen as 'active', so TypeScript can narrow the type from string to the string 'active'.

That can be handy for defining a union type from existing const values.

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