在web應用中，開發者經常會面臨字符的全角和半角轉換問題。那么什么是全角？什么是半角呢？簡單說就是中文系統用雙字節顯示漢字造成的與世界標準（由美國制訂的）不一的問題。
舉一個簡單的例子，中文逗號“，”就是全角的，相對的西文逗號“,”就是半角的。這樣的例子很多，比如“～”與“~”、“＋”與“+”等等。
那么，字符的全角和半角轉換有什么用處呢？一般說來，我們做的網頁都是中文的，要求用戶填入的信息也多是漢字，那么很多特殊字符都是全角的，比如說填寫多條信息，用逗號分隔，這里的逗號多半都是全角的。而提交到后臺（服務器端）處理，或者是查詢（構建select語句）或者是入庫（提交給DBMS），都是基于西文體系的，因此必須將這些字符轉換成對應的半角字符。
下面舉一個JavaScript的通用代碼例子，詳細介紹一下如何將這些全角字符轉換成半角字符。
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) //沒有查到對應的全角字符
{
result += ch;
}
}
return result;
}
代碼的原理就不詳細介紹了， src參數就是需要轉換的字符串，直接調用sbc2dbc函數就可以把src里面所有的全角字符轉換成半角字符。
為什么沒有半角字符轉換成全角字符的函數？呵呵，因為沒有想到那里會用到，如果你在開發中真有此需求，相信根據上面的源碼，你也可以寫出對應的轉換函數。祝你順利。