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方法调用 base64encode(str);

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//验证可用（此str已经转为ascii码，未转码的需先charCodeAt()）

function base64encode(str) {

    var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    var out, i, len;

    var c1, c2, c3;

    len = str.length;

    i = 0;

    out = "";

    while (i < len) {

        c1 = str[i++] & 0xff;//str.charCodeAt(i++)& 0xff;转ascii码

        if (i == len) {

            out += base64EncodeChars.charAt(c1 >> 2);

            out += base64EncodeChars.charAt((c1 & 0x3) << 4);

            out += "==";

            break;

        }

        c2 = str[i++];//str.charCodeAt(i++)

        if (i == len) {

            out += base64EncodeChars.charAt(c1 >> 2);

            out += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));

            out += base64EncodeChars.charAt((c2 & 0xF) << 2);

            out += "=";

            break;

        }

        c3 = str[i++];//str.charCodeAt(i++)

        out += base64EncodeChars.charAt(c1 >> 2);

        out += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));

        out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >> 6));

        out += base64EncodeChars.charAt(c3 & 0x3F);

    }

    return out;

}