用javascript限制input输入长度（考虑全角半角）

/*** 字符串求长度(全角) */String.prototype._getLength = function() { var str = this; var len = str.length; var reLen = 0; for (var i = 0; i < len; i++) { if (str.charCodeAt(i) < 27 || str.charCodeAt(i) > 126) { // 全角 reLen += 2; } else { reLen++; } } return reLen; }/*** 字符串截取部分(全角)* @param {Object} len*/String.prototype._cutString = function(len) { var str = this; var l = str.length; var rel = []; var tl = 0; for (var i = 0; i < l && tl < len; i++) { rel[i] = str[i]; if (str.charCodeAt(i) < 27 || str.charCodeAt(i) > 126) { tl += 2; } else { tl++; } } return rel.join("");}var DEFAULT_LENGTH = 20;function lengthValidate() { var len = DEFAULT_LENGTH - G("inputDemo").value._getLength(); if (len < 0) { G("inputDemo").value = G("inputDemo").value._cutString(C_LINE_F_LENGTH); G("inputDemo").maxLength = G("inputDemo").value.length - 1; len = 0; } else { G("inputDemo").maxLength = DEFAULT_LENGTH; } G("inputDemoInformation").innerHTML = "还可以输入" + len + "个字符";}