JS浅拷贝时遇到的一个问题

Question

   var extend = function (target, source) {      for (var key in source) {         target[key] = source[key];      }   };   var target = {name: 'target', skill: 'JS'};   var source = {name: 'source', friends: ['小明', '小宋']};   extend(target, source);       **疑问：为什么这里的 friends 提前变化了**   console.log(target.friends); // ["小明", "小宋"]   console.log(target); // {name: "source", skill: "JS", friends: [1, 2]}    target.name = 'tom';   target.skill = 'PHP';   target.friends = [1, 2];   console.log(target.friends); // [1, 2]   console.log(target); // {name: "tom", skill: "PHP", friends: [1, 2]}