Deep Merge

Requirements

• Export deepMerge(left, right) as named and default. Both root arguments must be plain objects, including null-prototype objects; otherwise throw TypeError. Return a fresh merged object.
• For keys present in both inputs: recursively merge two plain objects; concatenate two arrays in left-then-right order; otherwise replace the left value with the right value. An own right property containing undefined still overrides.
• Clone all plain-object and array containers in the result, including untouched branches and nested array elements. Do not mutate either input. Scalars and non-plain objects such as Date are atomic values and may retain their references. Repeated container references are cloned independently.
• Copy only own enumerable string object keys. Ignore inherited, symbol, and non-enumerable properties. For a merged object preserve the left object prototype; for a cloned replacement object preserve its source prototype. Array extra properties are outside scope, while length and holes are preserved.
• Support own keys such as __proto__, constructor, and prototype as ordinary data. Copy them with own-property definitions so setters or inherited properties do not affect the merge and no prototype pollution occurs.
• Inputs are acyclic data without getters. Property descriptors, class merging, circular graph handling, deletion markers, and configurable merge policies are outside scope.

Examples

Input: deepMerge({ user: { name: "Ada", score: 1 }, tags: ["a"] }, { user: { score: 2 }, tags: ["b"] });
Output: { user: { name: "Ada", score: 2 }, tags: ["a", "b"] }

Input: deepMerge({ setting: { x: 1 }, value: 3 }, { setting: null, value: undefined });
Output: { setting: null, value: undefined }