#741 Deep Merge Two Configs

A build tool merges a user's config file over its defaults. Write deepMerge(base, override) that returns a new object and changes neither input:

• Start from all of base's keys.

• For each key in override: if its value is null, the key is removed from the result. If both base and override have plain objects (not arrays) at that key, merge them with the same rules. Otherwise the override's value replaces the base's value (arrays are replaced, not combined).

Values in the result must not be shared with the inputs: copy nested objects and arrays.

Input: base = {"mode":"production","output":{"dir":"dist","clean":false},"plugins":["a"]}, override = {"output":{"clean":true},"plugins":["b"]}
Output: {"mode":"production","output":{"dir":"dist","clean":true},"plugins":["b"]}

Input: base = {"devtool":"source-map","cache":true}, override = {"devtool":null}
Output: {"cache":true}

Constraints

• Both inputs are JSON objects with at most 10^4 values in total.