js之数据类型（原始类型——数字，字符串，布尔值，空，undefiend，Symbol bigInt）

    script>
        　　　　  0.11;
        　　.ll111;
        　　1.4738223E-32 //1.4738223*10*(-23)(前一个*表示乖后一个*表示幂)
    script>

    script>
        console.log(Number.MAX_VALUE);//1.7976931348623157e+308
        console.log(Number.MIN_VALUE);//5e-324
        console.log(Number.MAX_SAFE_INTEGER);//9007199254740991
        console.log(Number.MIN_SAFE_INTEGER);//-9007199254740991

        //过isFinite()来确定一个数值是不是有穷的，包含着隐式类型转换Number()
        console.log(isFinite(Infinity))//false

        //isNaN() 来判断这个数字是不是NaN，包含着隐式类型转换Number()
        console.log(isNaN(‘hello‘));//true

        -0 === +0;//true;
        0 === -0;//true;
        0 === +0;//true
    script>

   script>
        console.log(Number(1));//1
        console.log(Number(true));//1
        console.log(Number(undefined));//NaN
        console.log(Number(NaN));//NaN
        console.log(Number(null));//0
        
        //转字符串
        console.log(Number(‘‘));//0
        console.log(Number(‘ ‘));//0
        console.log(Number(‘hello‘));//NaN

        //转对象
        console.log(Number([]));   //0
        console.log(Number([1,2]));//NaN
        // console.log(Number(其它对象)); //NaN
    script>

    script>
        console.log(parseInt(‘ 1234‘));//1234
        console.log(parseInt(‘ 1234fa‘));//1234
        console.log(parseInt(‘  a1234ps‘));//NaN
        console.log(parseInt([1,3])); //1 [1,3].toString()=>‘1,3‘
        console.log(parseInt(‘‘),parseInt(‘  ‘),parseInt({}));//NaN NaN NaN
    script>

    script>
      /*
       *意思是‘2AF5‘当做16进制，最后转成10进制
       * 现在要知道进制的转换：
       * 5*1+15*16^1+10*16^2+2*16^3=10997
       */
      parseInt("2AF5", 16);//10997
       /* 
         *3进制只能是0-2,所以查到3的时候，发现已经不是3进制的范围了，不再继续查找下去
         *意思是‘2‘当做3进制，最后转成10进制
         * 2*3^0 = 2
         */
      parseInt("231", 3);//2
        /* 
         * ‘234看做是5进制，转换为10进制‘
         * 4+3*5+2*5*5=69
         */
      parseInt(‘23461‘,5);//69
        //radix不在2-36之间，最后结果为NaN
      parseInt(1,1);//NaN
        //radix省略或者为0时，默认按10进制处理
      parseInt(1,0);//1
    script>

    script>
        console.log(parseFloat(‘ 1234ad‘));//1234
        console.log(parseFloat(‘ 1234‘));//1234
        console.log(parseFloat([1, 2]));//1
        console.log(parseFloat(true), parseFloat(false));
        console.log(parseFloat([]), parseFloat([2.1]), parseFloat([‘2.1px‘]));//NaN 2.1 2.1
        console.log(parseFloat({}), parseFloat(‘ ‘), parseFloat(‘‘));//NaN NaN NaN

 n = NaN;
    var s = ‘davina‘;//这个地方先用number()方法转一下，再用isNaN()方法
    var n1 = 12;
    console.log(isNaN(n), isNaN(s), isNaN(n1));//true true false

    script>
        console.log(Math.ceil(0.234)); //1
        console.log(Math.floor(0.789));//0
        console.log(Math.round(4.5));//5
        console.log(Math.abs(-9));//9
        console.log(Math.random())// 0.7037158897016866
    script>

    script>
        console.log(‘"hello world!!!"‘);//"hello world!!!"
        console.log("‘hello world!!!‘");//‘hello world!!!‘
        console.log(‘\‘hello world\‘!!!‘);//‘hello world‘!!!
        console.log(‘I don\‘t know.‘);//I don‘t know.

        //可以通过+运算符连接其他字符串来创建一个新字符串
        var lang = "java";
        lang = lang + "script";
        console.log(lang); //javascript

        //转字符串
        //a.toString()方法
        console.log(true.toString()); //true
        console.log([].toString());// 
        console.log({}.toString());//[object Object]
        console.log([1, 2, 3, 4].toString());//1,2,3,4
        console.log(new Date().toString());//Wed Nov 13 2019 12:06:11 GMT+0800 (中国标准时间)
        // console.log(null.toString()); 报错
        // console.log(undefined.toString()); 报错

        //b.String()方法
        var u = undefined;
        console.log(String(u)); //undefined

        var nul = null;
        console.log(String(nul));//null

        var num = 12;
        console.log(String(num));//12

        var b1 = true;
        var b2 = false;
        console.log(String(b1), String(b2));//true false
    script>

    
class="box">

  let title = "JS的学习要点",
    data = [
      {
        id: 0,
        title: "js",
      },{
        id: 1,
        title: "react",
      },
    ],
    box = document.querySelector(".box");

let str = "";
  str += ‘

‘;
  str += "
" + title + "";
  str += "
";
  data.forEach((item) => {
    str += "
" + item.title + "";
  });
  str == "";
  str += "
";
  box.innerHTML = str; 

  let str = `
  
class="learn">
        
${title}
        


         ${data.map((item) => {
           return `
${item.title}
`;
         }).join(‘‘)}
        



  `;
  box.innerHTML = str;

        //条件语句
        // if(a){
        //     条件为true时,执行这里的函数
        // }else{
        //     条件为false时，执行这里的函数
        // }

        //逻辑运算符 同时使用两个逻辑非操作符，可以将类型转换成布尔值
        console.log(!!1); //true
        console.log(!!0);//false
        console.log(!!‘‘)//false
        console.log(!!" ");//true

        //关系运算符用于测试两个值之间的关系，根据关系是否存在而返回true或者是false
        console.log(1 > 3);//false

        //转布尔值
        console.log(Boolean(undefined));//false
        console.log(Boolean(null));//false
        console.log(Boolean(NaN));//false
        console.log(Boolean(0));//false
        console.log(Boolean(‘‘));//false
        console.log(Boolean(‘ ‘));//true
        console.log(Boolean(false));//false
        console.log(Boolean(‘1234‘));//true

        //注意：空字符串转为false，空白字符串转为true
    script>

    script>
        console.log(document.getElementById(‘test‘));//null
        console.log(typeof null);//object
        console.log(null == undefined);//true

        console.log(null === null);//true
        console.log(undefined === null);//false
        console.log(‘null‘ === null);//false
    script>

    script>
        var i;
        console.log(typeof (i));// undefined
        var n = [];
        console.log(n.i);//undefined
        function fn() { };
        console.log(fn());//undefined
        function f(x) {
            return x;
        }
        console.log(f()); //undefined
    script>

 Symbol(10) === Symbol(10); //false 

 1 + 2);  //3
        console.log(1 + ‘a‘);  //1a
        console.log(true + 1); //2
        console.log(false + ‘1‘);  // false1
        console.log(false - true); //-1
        console.log(‘‘ - 1); //-1