JAVASCRIPT

deep merge nested objects javascript

function clone(obj, isStrictlySafe = false) {
  /* Clones an object. First attempt is safe. If it errors (e.g. from a circular reference),
        'isStrictlySafe' determines if error is thrown or an unsafe clone is returned. */
  try {
    return JSON.parse(JSON.stringify(obj));
  } catch(err) {
    if (isStrictlySafe) { throw new Error(err) }
    console.warn(`Unsafe clone of object`, obj);
    return {...obj};
  }
}

function merge(target, source, {isMutatingOk = false, isStrictlySafe = false} = {}) {
  /* Returns a deep merge of source into target.
        Does not mutate target unless isMutatingOk = true. */
  target = isMutatingOk ? target : clone(target, isStrictlySafe);
  for (const [key, val] of Object.entries(source)) {
    if (val !== null && typeof val === `object`) {
      if (target[key] === undefined) {
        target[key] = new val.__proto__.constructor();
      }
      /* even where isMutatingOk = false, recursive calls only work on clones, so they can always
            safely mutate --- saves unnecessary cloning */
      target[key] = merge(target[key], val, {isMutatingOk: true, isStrictlySafe}); 
    } else {
      target[key] = val;
    }
  }
  return target;
}