Implement deepMerge(target, source) — recursive merge

Question

Accepted Answer

**Approach:** For each key in source, if both values are plain objects, recurse; else assign.

```javascript
function isPlainObject(v) { return v !== null && typeof v === 'object' && Object.getPrototypeOf(v) === Object.prototype; }
function deepMerge(target, source) {
  const out = { ...target };
  for (const [k, v] of Object.entries(source)) {
    if (isPlainObject(v) && isPlainObject(out[k])) out[k] = deepMerge(out[k], v);
    else out[k] = v;
  }
  return out;
}

// Demo
console.log(deepMerge({a:{b:1, c:2}}, {a:{b:10, d:3}})); // {a:{b:10,c:2,d:3}}
console.log(deepMerge({a:[1,2]}, {a:[3]})); // {a:[3]} (replace)
console.log(deepMerge({a:1}, {b:2})); // {a:1,b:2}
function deepMergeMut(target, source) {
  for (const [k,v] of Object.entries(source)) {
    if (isPlainObject(v) && isPlainObject(target[k])) deepMergeMut(target[k], v);
    else target[k] = v;
  }
  return target;
}
```
Distinguish plain objects from Dates/RegExp/Maps — don’t deep-merge those.

Implement deepMerge(target, source) — recursive merge

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