typed-tuple

A function that returns a typed tuple from an array of literals.

Based on this excellent gist by Joe Calzaretta.

Install

npm install typed-tuple --save

then...

import tuple from 'tuple';

The Problem

Take the following string union type.

type Suit = 'hearts' | 'diamonds' | 'spades' | 'clubs';

This is great for restricting a variable to one of four suits. But what if you also want a list of all possible suits?

type Suit = 'hearts' | 'diamonds' | 'spades' | 'clubs';

const ALL_SUITS: Suit[] = ['hearts', 'diamonds', 'spades', 'clubs'];

The problem with this is ALL_SUITS is actually typed as Suit[]. But Suit[] can also mean any of the following values [], ['hearts'], ['hearts', 'hearts'], etc. We've also had to repeat ourselves.

We could explicitly type it like this:

type Suit = 'hearts' | 'diamonds' | 'spades' | 'clubs';

const allSuits: ['hearts', 'diamonds', 'spades', 'clubs'] = ['hearts', 'diamonds', 'spades', 'clubs'];

This gets the typing right, but we've made our code even less DRY - ack!

The Solution

We can use typed-tuple to reduce all this code:

import tuple from 'typed-tuple';

const ALL_SUITS = tuple('hearts', 'diamonds', 'spades', 'clubs');

type Suit = typeof ALL_SUITS[number];

Hooray! At last...

• ALL_SUITS is the correct type of ['hearts', 'diamonds', 'spades', 'clubs']
• Suit is the correct type of 'hearts' | 'diamonds' | 'spades' | 'clubs'
• And our code is now nice and DRY