SOURCE

if (typeof Object.assignWhenSame != 'function') {
  // Must be writable: true, enumerable: false, configurable: true
  Object.defineProperty(Object, "assignWhenSame", {
    value: function assignWhenSame(target, varArgs) { // .length of function is 2
      'use strict';
      // TypeError if undefined or null
      if (target == null) {
        throw new TypeError('Cannot convert undefined or null to object');
      }

      var to = new Object(target);

      for (var index = 1; index < arguments.length; index++) {
        var nextSource = arguments[index];

        // Skip over if undefined or null
        if (nextSource != null) {
          for (var nextKey in nextSource) {
            // Avoid bugs when hasOwnProperty is shadowed
            if (Object.prototype.hasOwnProperty.call(nextSource, nextKey) && Object.prototype.hasOwnProperty.call(target, nextKey)) {
              to[nextKey] = nextSource[nextKey];
            }
          }
        }
      }
      return to;
    },
    writable: true,
    configurable: true
  });
}

// test
console.log(Object.assignWhenSame({
  a: 1
},
{
  a: 2,
  b: 3
}));