For example:

  const themeMode = ['light', 'dark'] as const;
  type MainTheme = typeof themeMode[number];
  const values = [...themeMode];

  // iterate on themeMode as const assertion
  values.map((theme) => console.log(theme));

This answer is out of date; see @ggradnig's answer.

The best available workaround:

const furnitureObj = { chair: 1, table: 1, lamp: 1 };
type Furniture = keyof typeof furnitureObj;
const furniture = Object.keys(furnitureObj) as Furniture[];

Ideally we could do this:

const furniture = ['chair', 'table', 'lamp'];
type Furniture = typeof furniture[number];

Unfortunately, today furniture is inferred as string[], which means Furniture is now also a string.

We can enforce the typing as a literal with a manual annotation, but it brings back the duplication:

const furniture = ["chair", "table", "lamp"] as ["chair", "table", "lamp"];
type Furniture = typeof furniture[number];

TypeScript issue #10195 tracks the ability to hint to TypeScript that the list should be inferred as a static tuple and not string[], so maybe in the future this will be possible.

One built-in option would be to use an enum instead of the type and array approach.

export enum Stuff {
    something = 'something',
    else = 'else',
}

export const AVAILABLE_STUFF: Stuff[] = Object.values(Stuff);

Another option is to extract the type from the type of AVAILABLE_STUFF. To do this we must force the compiler to infer a tuple of string literals for AVAILABLE_STUFF. This can be done in 3.4 with as const or before 3.4 using an extra function. After AVAILABLE_STUFF is a tuple type we can just use a type query to get the type of the elements:

export const AVAILABLE_STUFF = (<T extends string[]>(...o: T)=> o)('something', 'else'); // typed as ["something", "else"]
// export const AVAILABLE_STUFF = ['something', 'else'] as const; // typed as ["something", "else"]  in 3.4
export type Stuff = typeof AVAILABLE_STUFF[number] //"something" | "else"

A few explanations of the above code. typeof AVAILABLE_STUFF gives us the type of the constant (["something", "else"]) to get the [number] is called a type query and will give us the type of an item in the tuple.

The (<T extends string[]>(...o: T)=> o) is just an IIFE we use to force the compiler to infer a string literal tuple type. It has to be generic as the compiler will only infer literal types and tuples in certain cases (a type parameter with a constraint of string being one of them). The as const version is what I would recommend using when it becomes available as it is more readable.