Object.leftJoin() implements a function to join 2 objects x and y based on the properties of x. However, if the properties of y are not shown in x then skip copy.

objectLeftJoin.js

var x = {a:1, b:2 };
var y = {a:5,     c:3, d:5};

Object.leftJoin = function(leftObj, rightObj) {
  var acceptKeyList = Object.keys(leftObj);
  var returnObj = {};
  
  for(var key of Object.keys(rightObj)) {
    if(acceptKeyList.indexOf(key) < 0) {
      Object.defineProperty(rightObj, key, {
        enumerable: false
      });
    }
  }
  return Object.assign(returnObj, leftObj, rightObj);
}

console.log(Object.leftJoin(x, y));