Get the type of the first and last tuple elements in TypeScript

How can we get the exact type of the first and last tuple elements in TypeScript?

Get the type of the first element

Getting the type of the first tuple element is actually very simple:

type First<T extends readonly unknown[]> = T[0]

const myArray = [1, 2, 3]

type FirstOfMyArray = First<typeof myArray> // 1 🎉

Note that if the array is empty, we’ll get undefined. If we’d like to alter this behavior, we can use a ternary and return whatever type we want:

type First<T extends readonly any[]> = T[0] extends undefined ? never : T[0]

Get type of the last element

Getting the last element of a tuple is a little bit complex. We’ll need to use a ternary with extends and infer combo:

type Last<T extends readonly any[]> = T extends readonly [...any, infer R] ? R : never

const test = [1, 2, 3, 4, 5] as const

type TestLast = Last<typeof test> // 5

In JavaScript, we cannot do the equivalent of [...any, infer R] because it would result in the syntax error “Rest element must be last element”. In this case, TypeScript is more flexible than JavaScript.

What about arrays?

Regarding arrays, TypeScript is much more limited in what it can actually infer. We can get the general type of an array using the same simple approach that we used on tuples:

type First<T extends any[]> = T[0]

const myArray = [1, 2, 3]

type FirstOfMyArray = First<typeof myArray> // number

If the array is of mixed type, TypeScript will only give us the union of all present types:

type First<T extends any[]> = T[0]

const myArray = [1, "orange", 3]

type FirstOfMyArray = First<typeof myArray> // string | number

As far as I know, it is not possible to get the type of the last element of an array.

If you find anything in this post that should be improved (either factually or in language), feel free to edit it on Github .