js学习笔记 -- 随记

• js不区分整数和浮点数，统一用Number表示，
• js'=='比较会自动转换类型，会产生奇怪结果，'==='不会转换比较类型，如果不一致返回false,因此js判断始终用'==='

• `` 保留换行，也可以有变量的占位符

  var name = '小明';
  var age = 20;
  var message = `你好, ${name}, 你今年${age}岁了!`;

• 用let替代var可以申明一个块级作用域的变量

      (function a() {
           for (var i=0; i<100; i++) {
           }
           console.log(i)//
       })();
       (function b() {
           for (let i=0; i<100; i++) {
           }
           console.log(i)//i is not defined
       })();

• JSON

      var xiaoming = {
          name:'lby',
          gender:'boy',
          age:null,
          height:null,
      };
      var filter = function (key, value) {
          if(value !==null)
          return value;
      }//过滤null值
      console.log(JSON.stringify(xiaoming,filter))//{"name":"lby","gender":"boy"}
      console.log(JSON.stringify(xiaoming))//{"name":"lby","gender":"boy","age":null,"height":null}

      var obj = JSON.parse('{"name":"lby","age":25}', function (key, value) {
          if (key === 'name') {
              return value + 'sssssss';
          }
          return value;
      });
      console.log(JSON.stringify(obj));//{"name":"lbysssssss","age":25}

• 解构赋值

  //数组
  let [x, y, z] = ['a', 'b','c'];
  console.log(x,y,z) //a b c
  let [x1, [y1, z1]] = ['a', ['b','c']];
  console.log(x1,y1,z1) //a b c

  //对象
  var p = {name:'lby',age:20,leg:{hair:50000}}
  var {name,age,leg:{hair}} = p
  console.log(name,age,hair)//lby 20 50000

  用解构互换2个值
  var x=1, y=2;
  [x, y] = [y, x]

  函数多参数
    var x = function ({a,b,c}) {
      console.log(a,b,c)//aa bb cc
    }
    x({a:'aa',b:'bb',c:'cc'})