数组迭代方法：map

   // map() 方法对数组每一项运行给定函数，返回每次函数调用的结果组成的数组
    let users = [
      { name: 't1', email: '[email protected]' },
      { name: 't2', email: '[email protected]' },
      { name: 't3', email: '[email protected]' },
    ];
    let userRes = users.map(ele => {
      return ele.email;
    });
    console.log('userRes', userRes);
    // userRes  ['[email protected]', '[email protected]', '[email protected]']