doc: add JSDoc

source/array-at.d.ts

@@ -7,6 +7,57 @@ import type {Subtract} from './subtract.d.ts';
 import type {Sum} from './sum.d.ts';
 import type {UnknownArray} from './unknown-array.d.ts';
 
+/**
+Return the element at the given index of the given array.
+
+Use-case: Get the element at a specific index of an array.
+
+Like [`Array#at()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at) but for types.
+
+@example
+```
+import type {ArrayAt} from 'type-fest';
+
+// Positive index
+type A = ArrayAt<['a', 'b', 'c', 'd'], 1>;
+//=> 'b'
+
+// Negative index
+type B = ArrayAt<['a', 'b', 'c', 'd'], -4>;
+//=> 'a'
+
+// Positive index with optional elements
+type C = ArrayAt<['a', 'b', 'c'?, 'd'?], 3>;
+//=> 'd' | undefined
+
+// Negative index with optional elements
+type D = ArrayAt<['a', 'b', 'c'?, 'd'?], -2>;
+//=> 'a' | 'b' | 'c'
+
+// Positive index with rest element and optional elements
+type E = ArrayAt<['a', 'b', 'c'?, ...number[]], 3>;
+//=> number | undefined
+
+// Negative index with rest element and optional elements
+type F = ArrayAt<['a', 'b', 'c'?, ...number[]], -1>;
+//=> 'b' | 'c' | number
+
+// Positive index with rest element in middle
+type G = ArrayAt<['a', 'b', 'c', ...number[], 'd', 'e'], 4>;
+//=> number | 'd' | 'e'
+
+// Negative index with rest element in middle
+type H = ArrayAt<['a', 'b', ...number[], 'c', 'd', 'e'], -5>;
+//=> 'a' | 'b' | number
+
+// Out-of-bounds positive index
+type I = ArrayAt<['a', 'b'], 5>;
+//=> undefined
+
+// Out-of-bounds negative index
+type J = ArrayAt<['a', 'b'], -3>;
+//=> undefined
+*/
 export type ArrayAt<TArray extends UnknownArray, Index extends number> =
 	TArray extends unknown // For distributing `Array_`
 		? Index extends unknown // For distributing `Index`