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}

@rmp0101

let target = {...existing,...newdata}; This code will be more than enough to merge the data by using javascript.

What part of the word deep you don't understand?

Very usefull! If someone wants to use more than of two objects you can combine this function with Array.reduce() and an array of objects.
[{}, {}, {}].reduce((ci, ni) => merge(ci, ni), {})

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.