JS Credit Card Mask

Description:

Usually when you buy something, you’re asked whether your credit card number, phone number or answer to your most secret question is still correct. However, since someone could look over your shoulder, you don’t want that shown on your screen. Instead, we mask it.

Your task is to write a function maskify, which changes all but the last four characters into ‘#’.

Examples

maskify("4556364607935616") == "############5616"
maskify(     "64607935616") ==      "#######5616"
maskify(               "1") ==                "1"
maskify(                "") ==                 ""

// "What was the name of your first pet?"
maskify("Skippy")                                   == "##ippy"
maskify("Nananananananananananananananana Batman!") == "####################################man!"

//No.1
function maskify(cc) {
  return cc.slice(0, -4).replace(/./g, '#') + cc.slice(-4);
  // .代表匹配除“\r\n”之外的任何单个字符
}

//No.2

function maskify(cc){
    return cc.replace(/.(?=....)/g,'#');
    //(?=pattern):正向肯定预查询，在任何匹配pattern的字符串开始处匹配查找字符串。这是一个非获取匹配，也就是说，该匹配不需要获取供以后使用。例如，"windows(?=95|98|NT|2000)"能匹配windows2000中的windows，但不能匹配windows3.1中的windows。预查不消耗字符，也就是说，在一个匹配发生后，在最后一次匹配之后立即开始下一次匹配的搜索，而不是从包含预查的字符之后开始。
}