jquery extend, minus jQuery

jquery-extend-without-jquery.js

var extend = function(){

  function isFunction(fn){ return typeof(fn) === "function" && fn.constructor === Function }
  function isArray(ar){ return ar instanceof Array }
  function isPlainObject(obj){ return typeof obj == 'object' && obj.constructor == Object }

  var options, name, src, copy, copyIsArray, clone, target = arguments[0] || {}, i = 1, length = arguments.length, deep = false;
  if(typeof target === "boolean"){
    deep = target;
    target = arguments[i] || {};
    i++;
  }
  if(typeof target !== "object" && !isFunction(target)) target = {};
  if(i === length){
    target = this;
    i--;
  }
  for(; i < length; i++){
    if((options = arguments[i]) != null){
      for(name in options){
        src  = target[name];
        copy = options[name];
        if(target === copy) continue;

        if(deep && copy &&(isPlainObject(copy) ||(copyIsArray = isArray(copy)))){
          if(!copyIsArray){
            copyIsArray = false;
            clone = src && isArray(src) ? src : [];
          } 
          else clone = src && isPlainObject(src) ? src : {};

          target[name] = extend(deep, clone, copy);
        }
	else if(copy !== undefined) target[name] = copy;
      }
    }
  }

  return target;
};

jquery-extends-without-jquery.ts

type Extendable = boolean | Array<unknown> | Record<string, unknown>;
type Extendables = Array<Extendable>;

function extend(..._: Extendables) {
  function isFunction(fn) {
    return typeof fn === "function" && fn.constructor === Function;
  }
  function isArray(ar) {
    return ar instanceof Array;
  }
  function isPlainObject(obj) {
    return typeof obj == "object" && obj.constructor == Object;
  }

  let options: unknown;
  let src: unknown;
  let copy: unknown;
  let copyIsArray = false;
  let clone: unknown;
  let target = arguments[0] || {};
  let i = 1;
  let deep = false;
  const length = arguments.length;

  if (typeof target === "boolean") {
    deep = target;
    target = arguments[i] || {};
    i++;
  }
  if (typeof target !== "object" && !isFunction(target)) target = {};
  if (i === length) {
    target = this;
    i--;
  }
  for (; i < length; i++) {
    if ((options = arguments[i]) != null) {
      for (const name in options) {
        src = target[name];
        copy = options[name];
        if (target === copy) continue;

        if (
          deep &&
          copy &&
          (isPlainObject(copy) || (copyIsArray = isArray(copy)))
        ) {
          if (!copyIsArray) {
            copyIsArray = false;
            clone = src && isArray(src) ? src : [];
          } else clone = src && isPlainObject(src) ? src : {};

          target[name] = extend(deep, clone as Extendable, copy as Extendable);
        } else if (copy !== undefined) target[name] = copy;
      }
    }
  }

  return target;
}