Question

我正在尝试编写代码来检查有效的用户名。我做错了什么？如果我输入我的用户名并以a-z开头，则说它无效。另外，我不能使用正则表达式。

var BR = "<br />";
var username = "";
var letter1;
var digit; 
letter1 = username.substr(0,1);

username = prompt("Please enter a your username:");

//First letter begin with a letter
if (letter1 >= "a" && letter1 <= "z") {
document.write("The first character is valid." + BR);
}
else {
document.write("The first character is not valid." + BR);
}

感谢您的帮助！

Answer 1

检查此小提琴http://jsfiddle.net/Dilip/2t9n6vpt/1/

var BR = "<br />";
var username = "";
var letter1;
var digit; 
letter1 = username.substr(0,1);

username = prompt("Please enter a your username:");

//First letter begin with a letter
var aCode = 'a'.charCodeAt(0);
var zCode = 'z'.charCodeAt(0);
if (username.charCodeAt(0) >= aCode && username.charCodeAt(0) <= zCode) {
document.write("The first character is valid." + BR);
}
else {
document.write("The first character is not valid." + BR);
}

username.charCodeAt（0）

将给出第一个字符的ASCII值。你可以用这个来检查。

Answer 2

把letter1 = username.substr（1,2）声明 AFTER 你从行读取用户名 username = prompt（“请输入您的用户名：”）;

在提示符弹出用户名后，您应该获得用户名的第一个字母。

Answer 3

切换“letter1 =”和“username =”语句的顺序。

Answer 4

将您的信件转换为ascii，然后按以下方式检查：https://jsfiddle.net/pyexm7us/2/

我也检查过大写字母：

的Javascript

var BR = "<br />";
var username = ""; 
var digit; 


username = prompt("Please enter a your username:");
var ascii = username.charCodeAt(0);

//First letter begin with a letter
if (ascii >= 97 && ascii <= 122 || ascii >= 65 && ascii <= 90) {
document.write("The first character is valid." + BR);
}
else {
document.write("The first character is not valid." + BR);
}

JAVASCRIPT - 检查第一个字母是A-Z

4 个答案: