How to delete recursively undefined properties from an object - while keeping the constructor chain?

Question

This is a question similar to How to remove undefined and null values from an object using lodash?. However, the solutions proposed there do not conserve the constructor. In add

Answer 1

You can use JSON.stringify(), JSON.parse(), RegExp.prototype.test()

JSON.parse(JSON.stringify(obj, function(a, b) {
  if (!/^_/.test(a) && b === undefined) {
    return null
  }
  return /^_/.test(a) && !b ? void 0 : b
}).replace(/null/g, '"undefined"'));

var obj = {key1 : 'value1', key2 : {key21 : 'value21', _key22: undefined}, key3: undefined, _key4 : undefined}

var res = JSON.stringify(obj, function(a, b) {
  if (!/^_/.test(a) && b === undefined) {
    return null
  }
  return /^_/.test(a) && !b ? void 0 : b
}).replace(/null/g, '"undefined"');

document.querySelector("pre").textContent = res;

res = JSON.parse(res);

console.log(res)