Typescript: Retrieve element type information from array type

Say I have some array type T[], is it possible to extract the type T within another alias / interface? For example my (fake) ideal code would be as follows:

// for illustration only...

type ArrayElement<T[]> = T;

// then, ArrayElement<string[]> === string

If no, are there general type theory reasons for not allowing such an operator? If no again, I might suggest it be added.

Thanks!

Solution 1:

Update: based on @jerico's answer below

The following type alias will return the type of the elements in an array or tuple:

type ArrayElement<ArrayType extends readonly unknown[]> = 
  ArrayType extends readonly (infer ElementType)[] ? ElementType : never;

So these examples would work:

type A = ArrayElement<string[]>; // string
type B = ArrayElement<readonly string[]>; // string
type C = ArrayElement<[string, number]>; // string | number
type D = ArrayElement<["foo", "bar"]>; // "foo" | "bar"
type E = ArrayElement<(P | (Q | R))[]>; // P | Q | R

type Error1 = ArrayElement<{ name: string }>; 
//                         ^^^^^^^^^^^^^^^^
// Error: Type '{ name: string; }' does not satisfy the constraint 'readonly unknown[]'.

Explanation

The type guard (the bit in the angle brackets) ArrayType extends readonly unknown[] says that we expect the type parameter ArrayType to be at least a readonly array (it also accepts a mutable array) so that we can look at its element type.

This prevents passing in a non-array value, as in the final example, which prevents ArrayElement ever returning never.

Note that readonly unknown[] is syntax added in TypeScript 3.4; for earlier versions use ReadonlyArray<unknown>.

On the right-hand side, the conditional expression asks the compiler to fill in the value of ElementType in the pattern readonly ElementType[] and return ElementType if it can, or never if it can't.

Since the type guard at the beginning means we will only ever be passed a value which matches this pattern, it's guaranteed always to match and never to return never.

Previous answer

type ArrayElement<ArrayType extends readonly unknown[]> = ArrayType[number];

Solution 2:

Since 2.1, typescript supports [ ] operator for types. The official name is indexed access types, also called lookup types, and it works like this:

type A = {a: string, b: number} [];

type AElement = A[0];

let e: AElement = {x: 0}; //error TS2322: Type '{ x: number; }' is not 
                       //assignable to type '{ a: string; b: number; }'