6、多个变量声明

javascript中使用到的变量，尽量在当前作用域的头部集体声明，变量声明应在一条声明语句中完成：只使用一个命令（var/let/const），各个变量名用逗号隔开。执行一条语句的速度高于执行多条语句的速度。
注：引用变量，以及常量尽量使用 const ，而普通变量则尽可能使用 let

反例

function func(){
    var count = 5;
  var color = 'blue';
  var value = [1, 2, 3];
  var now = new Date();
  var students = {
            name: '张三',
            num: '13356477'
          };
  //do something...
}
{
    let count = 5;
  let color = 'blue';
  let value = [1, 2, 3];
  let now = new Date();
  let students = {
            name: '张三',
            num: '13356477'
          };
}
const count = 5;
const color = 'blue';
const value = [1, 2, 3];
const now = new Date();
const students = {
        name: '张三',
        num: '13356477'
      };

正例

function func(){
    var count = 5,
            color = 'blue',
          value = [1, 2, 3],
          now = new Date(),
      students = {
            name: '张三',
            num: '13356477'
          };
  //do something...
}
{
    let count = 5,
          color = 'blue',
          value = [1, 2, 3],
          now = new Date(),
          students = {
            name: '张三',
            num: '13356477'
          };
}
const count = 5,
          color = 'blue',
          value = [1, 2, 3],
          now = new Date(),
          students = {
            name: '张三',
            num: '13356477'
          };