SOURCE

const str = 'we_are_family'
const toUp = (tar) => {
    let arr = tar.split('_')
    let str = arr[0]
    for(let i =1; i<arr.length; i++) {
        str += arr[i].substring(0,1).toUpperCase() + arr[i].substring(1)
    }
    return str
}

const toUp1 = (tar) => {
    let arr = tar.split('_')
    let str = ''
    arr.forEach((item, index) => {
        if (index > 0) {
            return str += item.replace(item[0], item[0].toUpperCase())
        }
        return str += item
    })
    return str
}

console.log(toUp(str))
console.log(toUp1(str))

const s = 'helloWorldOn'
const toline = (tar) => {
    let fn = (item) => {
        return '_'+ item.toLowerCase()
    }
    return s.replace((/[A-Z]/g), fn)
}
console.log(toline(s))
// console.log(str.substring(0,1))
// console.log(str.substring(1))
// console.log(str[0])