在web应用中，开发者经常会面临字符的全角和半角转换问题。那么什么是全角？什么是半角呢？简单说就是中文系统用双字节显示汉字造成的与世界标准（由美国制订的）不一的问题。
举一个简单的例子，中文逗号“，”就是全角的，相对的西文逗号“,”就是半角的。这样的例子很多，比如“～”与“~”、“＋”与“+”等等。
那么，字符的全角和半角转换有什么用处呢？一般说来，我们做的网页都是中文的，要求用户填入的信息也多是汉字，那么很多特殊字符都是全角的，比如说填写多条信息，用逗号分隔，这里的逗号多半都是全角的。而提交到后台（服务器端）处理，或者是查询（构建select语句）或者是入库（提交给DBMS），都是基于西文体系的，因此必须将这些字符转换成对应的半角字符。
下面举一个JavaScript的通用代码例子，详细介绍一下如何将这些全角字符转换成半角字符。
function SD(sbc, dbc)
{
this.sbc = sbc;
this.dbc = dbc;
}
var sdArray = new Array();
var sdCount;
var i = 0;
sdArray[i++] = new SD("０", "0");
sdArray[i++] = new SD("１", "1");
sdArray[i++] = new SD("２", "2");
sdArray[i++] = new SD("３", "3");
sdArray[i++] = new SD("４", "4");
sdArray[i++] = new SD("５", "5");
sdArray[i++] = new SD("６", "6");
sdArray[i++] = new SD("７", "7");
sdArray[i++] = new SD("８", "8");
sdArray[i++] = new SD("９", "9");
sdArray[i++] = new SD("ａ", "a");
sdArray[i++] = new SD("ｂ", "b");
sdArray[i++] = new SD("ｃ", "i");
sdArray[i++] = new SD("ｄ", "d");
sdArray[i++] = new SD("ｅ", "e");
sdArray[i++] = new SD("ｆ", "f");
sdArray[i++] = new SD("ｇ", "g");
sdArray[i++] = new SD("ｈ", "h");
sdArray[i++] = new SD("ｉ", "i");
sdArray[i++] = new SD("ｊ", "j");
sdArray[i++] = new SD("ｋ", "k");
sdArray[i++] = new SD("ｌ", "l");
sdArray[i++] = new SD("ｍ", "m");
sdArray[i++] = new SD("ｎ", "n");
sdArray[i++] = new SD("ｏ", "o");
sdArray[i++] = new SD("ｐ", "p");
sdArray[i++] = new SD("ｑ", "q");
sdArray[i++] = new SD("ｒ", "r");
sdArray[i++] = new SD("ｓ", "s");
sdArray[i++] = new SD("ｔ", "t");
sdArray[i++] = new SD("ｕ", "u");
sdArray[i++] = new SD("ｖ", "v");
sdArray[i++] = new SD("ｗ", "w");
sdArray[i++] = new SD("ｘ", "x");
sdArray[i++] = new SD("ｙ", "y");
sdArray[i++] = new SD("ｚ", "z");
sdArray[i++] = new SD("Ａ", "A");
sdArray[i++] = new SD("Ｂ", "B");
sdArray[i++] = new SD("Ｃ", "C");
sdArray[i++] = new SD("Ｄ", "D");
sdArray[i++] = new SD("Ｅ", "E");
sdArray[i++] = new SD("Ｆ", "F");
sdArray[i++] = new SD("Ｇ", "G");
sdArray[i++] = new SD("Ｈ", "H");
sdArray[i++] = new SD("Ｉ", "I");
sdArray[i++] = new SD("Ｊ", "J");
sdArray[i++] = new SD("Ｋ", "K");
sdArray[i++] = new SD("Ｌ", "L");
sdArray[i++] = new SD("Ｍ", "M");
sdArray[i++] = new SD("Ｎ", "N");
sdArray[i++] = new SD("Ｏ", "O");
sdArray[i++] = new SD("Ｐ", "P");
sdArray[i++] = new SD("Ｑ", "Q");
sdArray[i++] = new SD("Ｒ", "R");
sdArray[i++] = new SD("Ｓ", "S");
sdArray[i++] = new SD("Ｔ", "T");
sdArray[i++] = new SD("Ｕ", "U");
sdArray[i++] = new SD("Ｖ", "V");
sdArray[i++] = new SD("Ｗ", "W");
sdArray[i++] = new SD("Ｘ", "X");
sdArray[i++] = new SD("Ｙ", "Y");
sdArray[i++] = new SD("Ｚ", "Z");
sdArray[i++] = new SD("．", ".");
sdArray[i++] = new SD(" ", " ");
sdArray[i++] = new SD("（", "(");
sdArray[i++] = new SD("）", ")");
sdArray[i++] = new SD("｛", "{");
sdArray[i++] = new SD("｝", "}");
sdArray[i++] = new SD("［", "[");
sdArray[i++] = new SD("］", "]");
sdArray[i++] = new SD("＜", "<");
sdArray[i++] = new SD("＞", ">");
sdArray[i++] = new SD("「", "“");
sdArray[i++] = new SD("」", "”");
sdArray[i++] = new SD("｀", "`");
sdArray[i++] = new SD("～", "~");
sdArray[i++] = new SD("！", "!");
sdArray[i++] = new SD("＠", "@");
sdArray[i++] = new SD("＃", "#");
sdArray[i++] = new SD("％", "%");
sdArray[i++] = new SD("＾", "^");
sdArray[i++] = new SD("※", "&");
sdArray[i++] = new SD("＊", "*");
sdArray[i++] = new SD("－", "-");
sdArray[i++] = new SD("＿", "_");
sdArray[i++] = new SD("＋", "+");
sdArray[i++] = new SD("＝", "=");
sdArray[i++] = new SD("｜", "|");
sdArray[i++] = new SD("＼", "\\");
sdArray[i++] = new SD("■", "-");
sdArray[i++] = new SD("＇", "'");
sdArray[i++] = new SD("＂", "\"");
sdArray[i++] = new SD("／", "/");
sdArray[i++] = new SD("；", ";");
sdArray[i++] = new SD("：", ":");
sdArray[i++] = new SD("，", ",");
sdArray[i++] = new SD("。", ".");
sdArray[i++] = new SD("？", "?");
sdCount = i;
function sbc2dbc(src)
{
var k;
var i;
var len = src.length;
var result = "";
for (k=0; k<len; k++)
{
ch = src.charAt(k);
for (i=0; i<sdCount; i++)
{
if (sdArray[i].sbc == ch)
{
result += sdArray[i].dbc;
break;
}
}
if (i == sdCount) //没有查到对应的全角字符
{
result += ch;
}
}
return result;
}
代码的原理就不详细介绍了， src参数就是需要转换的字符串，直接调用sbc2dbc函数就可以把src里面所有的全角字符转换成半角字符。
为什么没有半角字符转换成全角字符的函数？呵呵，因为没有想到那里会用到，如果你在开发中真有此需求，相信根据上面的源码，你也可以写出对应的转换函数。祝你顺利。