1、加号左右两边只要有一个是字符串就是字符串的拼接
2、加号左右两边都是数字就是数字的四则运算
var str_1 = 'JavaSE'
console.log(str_1)
console.log(typeof str_1) //string
var str_2 = '-12.45'
console.log(str_2)
console.log(typeof str_2) //string
var num = 12.45
console.log(num)
var str_3 = 'fa$%& lse'
console.log(str_3)
console.log(str_1.length) //6
console.log(str_2.length) //5
console.log(str_3.length) //9
var strEmpty = ' '
console.log(strEmpty)
console.log(strEmpty.length)
var strNum1 = 12
var strWeb = 'web前端',
strJava = 'Java后端'
console.log('我喜欢的编程语言是:' + strWeb + strJava)
console.log(strJava + strNum1)
console.log(5 + strNum1) //17
console.log(isNaN(str_1)) //true
console.log(isNaN(str_2)) //false
console.log(isNaN(str_3)) //true
console.log(isNaN("")) //false --> 0