2.函数

1-1强制转换

1-1-1string boolean—>number

string—>number

         var str="10";
        console.log(str);//10
          var s="hello world"
         console.log(s);//NaN

boolean—>number

true --> 1
false -->0

1-1-2number string—>boolean

number—>boolean

        var a=3;
        var b=0;
        console.log(Boolean(a));//true
        console.log(Boolean(b));//false

string—>boolean

""-->false
        var str="hello world";
        var s="";
        console.log(Boolean(str));//true
        console.log(Boolean(s));//false

1-2自动转换

算术

        var a="10";
        var b=1;
        var c=true;
        console.log(a+b-c);//100
        console.log(a-b+c);//10
        console.log(a+c-b);//NaN

比较

        var a=true;
        var b="10";
        var c="hello";
        console.log(a>b);//false
        console.log(a>c);//false

1-3any-number

        var str="12.121212px";
        console.log(parseInt(str));//12
        console.log(parseFloat(str));//12.121212

1-4anystring

String();
toString();
+"";

1-5运算符和表达式

1-5-1算术运算

+，-，*，/,%
# 先将值转换为number(Number),再进行计算。
        var a=true;
        var b="13";
        var c=b-a;
        // var c=Number(b)-Number(a)
        console.log(c);  //12

1-5-2比较运算

        var a=true;
        var b="10";
        var c="hello";
        console.log(a>b);//false
        console.log(a>c);//false

1-5-3逻辑运算

        console.log(6>4 && 7>4);true
        console.log(4>3 && 6>7);false
        console.log(3>4 || 3>2);true
        console.log(3>4 || 3>5);false
        console.log(!(5>6));true

1-5-4赋值运算

        var a=4;
        // a=a+6;
         a+=6;
        console.log(a);//10
        a-=2;
        //a=a-2;
        console.log(a);//2

1-5-5三元表达式

var a=(5>4)?  "5>4" "4>5"
true:输出第一段"5>4"
false:输出第二段"4>5"

1-5-6++ —

        var a=10;
        var b=15;
        c=++a +2;
        d=b++ +4;
        console.log(c);13
        console.log(d);19
        console.log(b);16
        console.log(a);11

1-6箭头函数

        var go =function(){
            console.log("hello world")
        }
        go();  //hello world
        var show = ()=>{
            console.log("world")
        }
        show();  //world
        var test =()=>console.log("test");
        test();  //test

        var go =x=>console.log(x);  //10
        var sum =(x,y)=>{console.log(x+y)}; //7
        go(10);
        sum(3,4);

1-7奇怪的运算

        var a="21"+4-true+"3";
        console.log(a);//2133