What are “named or labeled tuples” in Typescript?

Question

Reading the changes in Typescript 4.0, I found the new feature:

Labeled Tuple Elements

I thought elements could either be indexed by numbers (like in tuples and lists) or by keys (like in dicts). I don't expected they could be indexed both ways.

my questions is:

• Why/when should I use named tuples instead of normal tuples?

Triplet

They can't be "indexed both ways", the name labels are metadata. The actual value at runtime is still just an array, TS only exists at compile time. – Pekan

Answer 1

This is purely for documentation purposes, it has no semantics. They are just a way of putting names in the type signature--they type check identically to tuples without names, and the runtime behavior is identical as well.

While these have no impact on type-checking, the lack of labels on tuple positions can make them harder to use – harder to communicate our intent.

Emphasis added.

Use them whenever you want to document what the names of elements in a tuple are in the type signature of a function that uses them.

Example

You might have a Range type, which is [start, end]:

type Range = [start: number, end: number];

Or your Range type might be [start, length]:

type Range = [start: number, length: number];

Or you could use unnamed tuples:

type Range = [number, number];

These three definitions have identical semantics as far as TypeScript is concerned. You can access the members through destructuring by array access (e.g. arr[0]), just like any other array--they are not special. You cannot access the elements by name... again, these are just ordinary JavaScript array objects.

Digest

They do have some impact on type checking if you spread them, though. Using [x?: number] as the arguments of a function will make the first param optional, whereas using [number | undefined] will not. – Conviction

@m93a, you have to use the right syntax. If you want an optional entry in an unlabeled tuple, you do [number?]. – Argufy

How do I access the name of an element of such a tuple? – Bobker

@sayandcode: You cannot access the name. – Digest