My least favorite thing about ES5

assign.js

// I want to add multiple properties to a target object in one statement without adding extra variables to the current scope

// This can be done in ES6 with Object.assign:

var targetObj = {};

Object.assign({
  a: 1,
  b: 2
}, targetObj);

// Using ES5 it is impossible without an intermediary variable for the source object:

(function() {
  var sourceObj  = {
    a: 1,
    b: 2
  };
  Object.keys(sourceObj).forEach(function(key) {
    targetObj[key] = sourceObj[key];
  });
})();