Deep Map

Requirements

• Export deepMap(input, mapper) as named and default. Recursively visit arrays and plain objects, including objects with a null prototype. Apply mapper(value, path) only to leaves.
• A leaf is any value that is not an array or plain object, including null, undefined, functions, and non-plain objects. Empty objects and arrays remain empty containers and do not invoke mapper.
• path is a fresh array of string property names and numeric array indexes from the root. A root leaf gets an empty path. Visit object keys in Object.keys order and array indexes in ascending order.
• Return new containers without mutating the input. Preserve array length and holes, and preserve plain-object/null-object prototypes. Copy only own enumerable string properties of objects; array extra properties, symbols, and descriptors are outside scope.
• Mapped leaf results are inserted unchanged, even when they are objects or arrays; do not recursively remap them. Shared source containers are mapped independently, rather than preserving aliases.
• The input is acyclic and uses data properties, not getters. Mapper must be pure. Support own keys such as __proto__ without changing the output prototype or polluting other objects. TypeScript returns unknown because leaf mapping can change value types.

Examples

Input: deepMap({ score: 2, items: [3, null] }, value => typeof value === "number" ? value * 2 : value);
Output: { score: 4, items: [6, null] }

Input: deepMap({ user: { name: "Ada" } }, (value, path) => path.join(".") + ":" + value);
Output: { user: { name: "user.name:Ada" } }