输入'niHao aAz'实现输出为'ni_hao a_az'

var b = 'niHao aAz' //输出ni_hao

var a = b.split('')

var str = [];

for (var i = 0; i < a.length; i++) {

if (a[i] >= 'A' && a[i] <= 'Z') {

str.push('_')

str.push(a[i].toLowerCase())

} else {

str.push(a[i])

}

}

console.log(str.join(''))