Deep-Merge JavaScript objects with ES6

deep-merge.js

// ⚠ IMPORTANT: this is old and doesn't work for many different edge cases but I'll keep it as-is for any of you want it
// ⚠ IMPORTANT: you can find more robust versions in the comments or use a library implementation such as lodash's `merge`

// Merge a `source` object to a `target` recursively
const merge = (target, source) => {
  // Iterate through `source` properties and if an `Object` set property to merge of `target` and `source` properties
  for (const key of Object.keys(source)) {
    if (source[key] instanceof Object) Object.assign(source[key], merge(target[key], source[key]))
  }

  // Join `target` and modified `source`
  Object.assign(target || {}, source)
  return target
}

deep-merge.min.js

const merge=(t,s)=>{const o=Object,a=o.assign;for(const k of o.keys(s))s[k]instanceof o&&a(s[k],merge(t[k],s[k]));return a(t||{},s),t}

Non-mutating deep merge and copy, making use of the newish structuredClone function for the copying

function deepMerge(target, source) {
  const result = { ...target, ...source };
  for (const key of Object.keys(result)) {
    result[key] =
      typeof target[key] == 'object' && typeof source[key] == 'object'
        ? deepMerge(target[key], source[key])
        : structuredClone(result[key]);
  }
  return result;
}

(some more care would be needed if you need to handle Arrays)

note that structuredClone still requires you to do prototype assignment for classed/prototyped objects, though. That doesn't come free.