JavaScript基本数据类型转换你了解嘛

Number('1')   // 1
Number(true)  // 1
Number('123s') // NaN
Number({})  //NaN

parseInt('2') //2
parseInt('2',10) // 2
parseInt('2',2)  // NaN
parseInt('a123')  // NaN  如果第一个字符不是数字或者符号就返回NaN
parseInt('123a')  // 123

parseFloat('123a')	//123
parseFloat('123a.01')	//123
parseFloat('123.01')	//123.01
parseFloat('123.01.1')	//123.01

let str = '123'-str// -123str+1 // '1231'+str+1 // 124let res = str - 1 //122let str = '123'
-str	// -123
str+1 // '1231'
+str+1 // 124
let res = str - 1 //122

Number(123).toString()	//'123'
[].toString()	//''
true.toString()	//'true'

String(123)	//'123'
String(true)	//'true'
String([])	//''
String(null)	//'null'
String(undefined)	//'undefined'
String({})	//'[object Object]'

let a = 1
a + ''	// '1'

Boolean('') //false
Boolean(0) //false
Boolean(1) //true
Boolean(null) //false
Boolean(undefined) //false
Boolean(NaN) //false
Boolean({}) //true
Boolean([]) //true

let a
if(a) {
  //...   //这里a为undefined，会转为false，所以该条件语句内部不会执行
}

let str = '111'
console.log(!!str) // true