mergeIn.js

const isObject = o => typeof o === 'object' && o !== null;

const merge = (source, key, update) =>
  Array.isArray(source)
    ? [...source.slice(0, key > 0 ? key - 1 : 0), update, ...source.slice(key + 1)]
    : { ...source, [key]: update };

const mergeIn = (source, target, update) =>
  source === target
    ? Array.isArray(source) ? update : { ...source, ...update }
    : Object.keys(source).reduce((acc, key) => {
        const value = source[key];
        if (value === target) return merge(source, key, value);
        if (isObject(value)) return mergeIn(value, target, update);
        return source;
      }, source);

// TODO: `changeIn(source, target, field, value)`