字符串转换base64的方法

base64_encode(str){
        var c1, c2, c3;
        var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
        var i = 0,
            len = str.length,
            string = '';

        while (i < len) {
            c1 = str.charCodeAt(i++) & 0xff;
            if (i == len) {
                string += base64EncodeChars.charAt(c1 >> 2);
                string += base64EncodeChars.charAt((c1 & 0x3) << 4);
                string += "==";
                break;
            }
            c2 = str.charCodeAt(i++);
            if (i == len) {
                string += base64EncodeChars.charAt(c1 >> 2);
                string += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));
                string += base64EncodeChars.charAt((c2 & 0xF) << 2);
                string += "=";
                break;
            }
            c3 = str.charCodeAt(i++);
            string += base64EncodeChars.charAt(c1 >> 2);
            string += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));
            string += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >> 6));
            string += base64EncodeChars.charAt(c3 & 0x3F)
        }
        return string
    }