There is an undefined problem at the front end
1. There is only a statement, no assignment
2. The parameters in the method are not passed in
3. Calling an attribute that is not defined in an object
Number conversion
console.log(Number(true));//1
console.log(Number(null));//0
console.log(Number(undefined));//NaN
console.log(Number("0.2"));//0.2
console.log(Number(""));//0
console.log(Number("AA"));//NaN
///Number里面的方法
console.log(0.000011226.toExponential());//科学计数法
console.log(12.22.toFixed());//四舍五入
console.log(Number.isInteger(10));//判断是否为整数
console.log(isNaN('AAA'));//isNaN=Is not a number
pressInt pressFloat
console.log(parseInt("22"));//22
console.log(parseInt("202pxp"));//202
console.log(parseInt("cc12"));//Nan
console.log(parseFloat("1530x"));//1530
console.log(parseFloat("0123x"));//123
console.log(parseFloat("c1530x"));//NaN
“” ‘’ ``
let a="aaa";
console.log(a);
let b='bbb'
console.log(b);
let c=`022222220000${a}33333333${b}`;
console.log(c);
let d="t'cant bo fffd";
console.log(d);
let e='fgsg"fgggg"';
console.log(e);
JS three special reference types
let s='aa a';
let s1=new String (' ');
console.log(s.split(' '));
console.log(typeof(s));
console.log(typeof(s1));
var ss="hello world";
console.log(ss.length);
console.log(ss.charAt(1));//位置为1处的字符
console.log(ss.charCodeAt(1));//位置为1处的字符编码
console.log(ss.concat("aa"));
console.log(ss.slice(3));