How to create a type from a variable in Typescript?

Question

I need to create a type that, according to the variable value, uses a specific type.

I have the following constant variables:

const NEW = 'NEW_PROJECT';
const DELETE = 'DELETE_PROJECT';

The code is working with the following type, defined with string types:

type ProjectsActionsType =
  | { type: 'NEW_PROJECT'; payload: { id: string } }
  | { type: 'DELETE_PROJECT'; payload: { id: string } };

But I want to use the constant variable type instead of string types.

I tried this:

type ProjectsActionsType =
  | { type: NEW; payload: { id: string } }
  | { type: DELETE; payload: { id: string } };

But it didn’t work; I received this error:

'NEW' refers to a value, but is being used as a type here. ts(2749)

How can I use variables in Typescript type settings values?

Answer 1

At type level, the operator typeof returns the type (if present) of its operand value.

For example:

const foo = "FOO_STRING";
type Derived = typeof foo; // tipo inferido é: "FOO_STRING"

See more on typescript documentation.

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Be careful not to confuse with the operator typeof javascript, which does not operate at type level, but Runtime.