Pass non-plain object values (Date, Map, ...) through in change-case/keys

[yfwmaniish]

Closes #356.

Problem

change-case/keys recurses into every value that is typeof === "object", including built-ins like Date, Map, Set, and RegExp. For those it builds a shallow clone with Object.create(Object.getPrototypeOf(object)) and copies own enumerable keys — but built-ins carry their state in internal slots, not enumerable properties, so the "clone" is a broken husk with the right prototype and none of the internals.

For a Date this is easy to hit (from #356):

import { camelCase } from "change-case/keys";

const out = camelCase({ created_at: new Date() }, 2);
out.createdAt.toISOString();
// ❌ TypeError: Method Date.prototype.toISOString called on incompatible receiver [object Object]

out.createdAt is Date {} — a prototype-only object with no [[DateValue]]. The same breakage applies to Map, Set, RegExp, and any class instance that relies on internal slots.

Fix

Only recurse into arrays and plain objects; pass any other object through unchanged as a value. Plain-object detection is a prototype check (Object.prototype or null), so Object.create(null) bags still get transformed, and nested plain objects/arrays are unaffected.

const isPlainObject = (object: unknown): boolean => {
  if (object === null || typeof object !== "object") return false;
  const prototype = Object.getPrototypeOf(object);
  return prototype === null || prototype === Object.prototype;
};

-    if (depth === 0 || !isObject(object)) return object;
+    if (depth === 0) return object;

     if (Array.isArray(object)) {
       return object.map((item) => changeKeys(item, depth - 1, options));
     }

+    if (!isPlainObject(object)) return object;

Behavior

Input value	Before	After
plain object	keys transformed	keys transformed (unchanged)
array	mapped	mapped (unchanged)
Object.create(null)	keys transformed	keys transformed (unchanged)
Date / Map / Set / RegExp	broken empty clone	passed through as-is

Tests

Added a keys.spec.ts case asserting Date, Map, and RegExp values are passed through by reference and remain functional (toISOString() works). Full change-case suite passes (28 tests), typecheck and Prettier clean.

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