Immutability and depth

Every built-in transform returns a new top-level value. The original object or array remains unchanged.

const source = {user_id: 1, profile: {theme: 'dark'}};
const result = rename({user_id: 'id'})(source);

source.user_id; // 1
result.id;      // 1

Transforms are shallow. Nested object references are preserved unless you explicitly target one with at. merge is also shallow, so a right-hand nested object replaces the left-hand nested object rather than combining their children.

at uses structural sharing: it clones only the objects from the root through the transformed target. Sibling branches keep their original references. Missing paths and non-object targets return the original root reference unchanged.

This makes each operation predictable and keeps hidden recursive work out of large API responses.