Convert an Enum or an object to a literal union type

This is a TypeScript goodie! I use it quite often for states.

Enum to Union type

If you want to convert an enum like

enum State {
  SUCCESS = 'success',
  ERROR = 'error',
  PENDING = 'pending',
}

to an union of string literal types you can use Template Literal Types to do that.

// Result: type Status = 'success' | 'error' | 'pending';
type Status = `${State}`;

Object to Union type

A normal object has initially just strings as value.

/* 
  const State: {
    SUCCESS: string;
    ERROR: string;
    PENDING: string;
  } 
*/

const State = {
  SUCCESS: 'success',
  ERROR: 'error',
  PENDING: 'pending',
};

If you want to use an object to fixate the values you can convert the values of the object from string to a literal type (‘success’, ‘error’ and ‘pending’) by using const assertions. Add as const behind the object.

/* 
  const State: {
    readonly SUCCESS: "success";
    readonly ERROR: "error";
    readonly PENDING: "pending";
  }
*/

const State = {
  SUCCESS: 'success',
  ERROR: 'error',
  PENDING: 'pending',
} as const;

Then you can convert the values to an union of string literal types with typeof State and using keyof typeof State as a key.

/* 
  typeof State = type StateObject = {
    readonly SUCCESS: "success";
    readonly ERROR: "error";
    readonly PENDING: "pending";
  }

  keyof typeof State = "SUCCESS" | "ERROR" | "PENDING"
*/

// Result: type Status = 'success' | 'error' | 'pending';
type Status = (typeof State)[keyof typeof State];