mapKeys / mapValues

mapValues and mapKeys are Array.prototype.map for objects. mapValues transforms every value while keeping the keys; mapKeys transforms every key while keeping the values. Both return a new object with the same number of entries, just reshaped.

Implement both. Each takes (obj, iteratee), where iteratee(value, key) produces the new value (for mapValues) or the new key (for mapKeys). Return a new object; don't mutate the input.

Signature

function mapValues(obj, iteratee) {} // same keys, iteratee(value, key) as each value
function mapKeys(obj, iteratee) {}   // same values, iteratee(value, key) as each key

Examples

mapValues({ a: 1, b: 2 }, (v) => v * 10);        // { a: 10, b: 20 }
mapKeys({ a: 1, b: 2 }, (v, k) => k.toUpperCase()); // { A: 1, B: 2 }

// The iteratee gets both value and key:
mapValues({ a: 1 }, (v, k) => `${k}=${v}`); // { a: 'a=1' }
mapKeys({ a: 1, b: 2 }, (v) => `k${v}`);     // { k1: 1, k2: 2 }

Notes

• mapValues keeps keys; mapKeys keeps values — each changes exactly one axis.
• Iteratee signature — (value, key), in that order (value first), matching lodash.
• Key collisions — if two keys map to the same new key in mapKeys, the later entry wins.
• Own enumerable only — transform the object's own enumerable properties; skip inherited ones.
• New object, no mutation — return a fresh object; leave the input alone.