The two-step entries then fromEntries pipeline is the most readable functor-like transform in plain JavaScript. Passing both value and key to the callback (in that order, matching Array#map) lets the caller branch on the key when needed without forcing a separate helper. This runs in O(n) time and allocates two intermediate arrays plus the result; for hot paths with thousands of keys, prefer the manual loop variant in the next accordion. Use this version for one-shot transforms where readability beats raw throughput.

When the same transform fires on every request (think feature-flag rollout or per-row formatters), the manual loop avoids two array allocations and is roughly twice as fast in micro-benchmarks. Object.hasOwn(obj, key) filters out inherited properties safely, replacing the older obj.hasOwnProperty.call(...) dance. The for..in loop is fine here because we re-check ownership on every iteration. Reach for this version inside ETL pipelines and renderer hot loops; stick with the fromEntries form everywhere else.

Renaming keys (mapKeys) is the natural sibling of mapValues, and once you have both you usually want a mapEntries that returns a fresh [key, value] tuple per pair. The snake-to-camel example is the most common real-world use, often paired with API responses from Python or Rust backends. Watch for collisions: if fn returns the same key for two distinct inputs, the later one silently wins, which is rarely what you want. When that's a risk, accumulate values into an array per key instead of overwriting.