Error compiling code for arduino uno rfid-rc522

I got a error compiling code for my rfid-rc522 and I don't know what to do, I don't know C and I am a nob to arduino. I am using a linux system to do this in also if that helps

Arduino: 1.8.19 (Linux), Board: "Arduino Uno"

/usr/share/arduino/arduino-builder -dump-prefs -logger=machine -hardware /usr/share/arduino/hardware -tools /usr/share/arduino/tools-builder -libraries /home/the-beast/Arduino/libraries -fqbn=archlinux-arduino:avr:uno -vid-pid=2341_0043 -ide-version=10819 -build-path /tmp/arduino_build_884578 -warnings=none -build-cache /tmp/arduino_cache_250380 -prefs=build.warn_data_percentage=75 -verbose /tmp/arduino_modified_sketch_782354/DumpInfo.ino
/usr/share/arduino/arduino-builder -compile -logger=machine -hardware /usr/share/arduino/hardware -tools /usr/share/arduino/tools-builder -libraries /home/the-beast/Arduino/libraries -fqbn=archlinux-arduino:avr:uno -vid-pid=2341_0043 -ide-version=10819 -build-path /tmp/arduino_build_884578 -warnings=none -build-cache /tmp/arduino_cache_250380 -prefs=build.warn_data_percentage=75 -verbose /tmp/arduino_modified_sketch_782354/DumpInfo.ino
Using board 'uno' from platform in folder: /usr/share/arduino/hardware/archlinux-arduino/avr
Using core 'arduino' from platform in folder: /usr/share/arduino/hardware/archlinux-arduino/avr
Detecting libraries used...
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard /tmp/arduino_build_884578/sketch/DumpInfo.ino.cpp -o /dev/null
Alternatives for SPI.h: [SPI@1.0]
ResolveLibrary(SPI.h)
  -> candidates: [SPI@1.0]
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src /tmp/arduino_build_884578/sketch/DumpInfo.ino.cpp -o /dev/null
Alternatives for MFRC522.h: [MFRC522@1.4.10 MFRC522-spi-i2c-uart-async@1.5.1]
ResolveLibrary(MFRC522.h)
  -> candidates: [MFRC522@1.4.10 MFRC522-spi-i2c-uart-async@1.5.1]
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /tmp/arduino_build_884578/sketch/DumpInfo.ino.cpp -o /dev/null
Using cached library dependencies for file: /usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src/SPI.cpp
Using cached library dependencies for file: /home/the-beast/Arduino/libraries/MFRC522/src/MFRC522.cpp
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp -o /dev/null
Generating function prototypes...
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /tmp/arduino_build_884578/sketch/DumpInfo.ino.cpp -o /tmp/arduino_build_884578/preproc/ctags_target_for_gcc_minus_e.cpp
/usr/bin/arduino-ctags -u --language-force=c++ -f - --c++-kinds=svpf --fields=KSTtzns --line-directives /tmp/arduino_build_884578/preproc/ctags_target_for_gcc_minus_e.cpp
Compiling sketch...
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /tmp/arduino_build_884578/sketch/DumpInfo.ino.cpp -o /tmp/arduino_build_884578/sketch/DumpInfo.ino.cpp.o
Compiling libraries...
Compiling library "SPI"
Using previously compiled file: /tmp/arduino_build_884578/libraries/SPI/SPI.cpp.o
Compiling library "MFRC522"
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp -o /tmp/arduino_build_884578/libraries/MFRC522/MFRC522Extended.cpp.o
Using previously compiled file: /tmp/arduino_build_884578/libraries/MFRC522/MFRC522.cpp.o
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp: In member function 'MFRC522::StatusCode MFRC522Extended::TCL_Transceive(TagInfo*, byte*, byte, byte*, byte*)':
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp:824:34: error: ordered comparison of pointer with integer zero ('byte*' {aka 'unsigned char*'} and 'int')
  824 |         if (backData && (backLen > 0)) {
      |                          ~~~~~~~~^~~
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp:847:42: error: ordered comparison of pointer with integer zero ('byte*' {aka 'unsigned char*'} and 'int')
  847 |                 if (backData && (backLen > 0)) {
      |                                  ~~~~~~~~^~~
Multiple libraries were found for "MFRC522.h"
 Used: /home/the-beast/Arduino/libraries/MFRC522
 Not used: /home/the-beast/Arduino/libraries/MFRC522-spi-i2c-uart-async
Using library SPI at version 1.0 in folder: /usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI 
Using library MFRC522 at version 1.4.10 in folder: /home/the-beast/Arduino/libraries/MFRC522 
exit status 1
Error compiling for board Arduino Uno.

Welcome to the forum

Start by posting you code here, using code tags when you do

In my experience the easiest way to tidy up the code and add the code tags is as follows

Start by tidying up your code by using Tools/Auto Format in the IDE to make it easier to read. Then use Edit/Copy for Forum and paste what was copied in a new reply. Code tags will have been added to the code to make it easy to read in the forum thus making it easier to provide help.

See here, looks like the same problem you're experiencing:

It seems to be a library bug, that source code should be changed from:
if (backData && (backLen > 0)) {
to
if (backData && backlen != nullptr)) {

I saw that post and it didn't help me at all, i didn't understand what I had to do

/*
 * --------------------------------------------------------------------------------------------------------------------
 * Example sketch/program showing how to read data from a PICC to serial.
 * --------------------------------------------------------------------------------------------------------------------
 * This is a MFRC522 library example; for further details and other examples see: https://github.com/miguelbalboa/rfid
 * 
 * Example sketch/program showing how to read data from a PICC (that is: a RFID Tag or Card) using a MFRC522 based RFID
 * Reader on the Arduino SPI interface.
 * 
 * When the Arduino and the MFRC522 module are connected (see the pin layout below), load this sketch into Arduino IDE
 * then verify/compile and upload it. To see the output: use Tools, Serial Monitor of the IDE (hit Ctrl+Shft+M). When
 * you present a PICC (that is: a RFID Tag or Card) at reading distance of the MFRC522 Reader/PCD, the serial output
 * will show the ID/UID, type and any data blocks it can read. Note: you may see "Timeout in communication" messages
 * when removing the PICC from reading distance too early.
 * 
 * If your reader supports it, this sketch/program will read all the PICCs presented (that is: multiple tag reading).
 * So if you stack two or more PICCs on top of each other and present them to the reader, it will first output all
 * details of the first and then the next PICC. Note that this may take some time as all data blocks are dumped, so
 * keep the PICCs at reading distance until complete.
 * 
 * @license Released into the public domain.
 * 
 * Typical pin layout used:
 * -----------------------------------------------------------------------------------------
 *             MFRC522      Arduino       Arduino   Arduino    Arduino          Arduino
 *             Reader/PCD   Uno/101       Mega      Nano v3    Leonardo/Micro   Pro Micro
 * Signal      Pin          Pin           Pin       Pin        Pin              Pin
 * -----------------------------------------------------------------------------------------
 * RST/Reset   RST          9             5         D9         RESET/ICSP-5     RST
 * SPI SS      SDA(SS)      10            53        D10        10               10
 * SPI MOSI    MOSI         11 / ICSP-4   51        D11        ICSP-4           16
 * SPI MISO    MISO         12 / ICSP-1   50        D12        ICSP-1           14
 * SPI SCK     SCK          13 / ICSP-3   52        D13        ICSP-3           15
 *
 * More pin layouts for other boards can be found here: https://github.com/miguelbalboa/rfid#pin-layout
 */

#include <SPI.h>
#include <MFRC522.h>

#define RST_PIN         9          // Configurable, see typical pin layout above
#define SS_PIN          10         // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN);  // Create MFRC522 instance

void setup() {
	Serial.begin(9600);		// Initialize serial communications with the PC
	while (!Serial);		// Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
	SPI.begin();			// Init SPI bus
	mfrc522.PCD_Init();		// Init MFRC522
	delay(4);				// Optional delay. Some board do need more time after init to be ready, see Readme
	mfrc522.PCD_DumpVersionToSerial();	// Show details of PCD - MFRC522 Card Reader details
	Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));
}

void loop() {
	// Reset the loop if no new card present on the sensor/reader. This saves the entire process when idle.
	if ( ! mfrc522.PICC_IsNewCardPresent()) {
		return;
	}

	// Select one of the cards
	if ( ! mfrc522.PICC_ReadCardSerial()) {
		return;
	}

	// Dump debug info about the card; PICC_HaltA() is automatically called
	mfrc522.PICC_DumpToSerial(&(mfrc522.uid));
}

Next to the end there's how someone solved it, as I wrote:

Give it a try, I can't test that.

Have you got one library folder inside the another, duplicate library folder? I find that downloading and manually extracting with WinRar (or whatever Linux is using, maybe) that sometimes it will extract this way.

I don't have if (backData && (backLen > 0)) { in my program should I just add if (backData && backlen != nullptr)) { to the end?

I installed my library from sketch include library, I didn't install anything external into Arduino IDE

What part of "it seems to be a library bug" was unclear?
How to fix it is described on Github, the page I linked you before.

If you aren't experienced enough, just search for the "MFRC522Extended.cpp" file, you can find two lines (#824 and #847) with the problem, the two you can read on your compiler output:
image

Then change both lines to:

if (backData && backLen) {

and try compiling again.

Obviously if you later will update or upgrade the library, you'll lose those changes (but hope they fixed it in the meanwhile) so don't do it unless in the future will be strictly needed for you.

Navigate to the /libraries/MFRC522/src/MFRC522Extended.cpp path within the sketchbook folder for your project. From there search for the lines of code that say if (backData && (backLen > 0)) { (should be lines #824 and #847 ) and replace them with (backData && backlen != nullptr)) {

okay I just changed those lines but I'm getting the same error it seems, I tried closing and opening again and it didn't help

this is the error I just got after doing that

Arduino: 1.8.19 (Linux), Board: "Arduino Uno"

/usr/share/arduino/arduino-builder -dump-prefs -logger=machine -hardware /usr/share/arduino/hardware -tools /usr/share/arduino/tools-builder -libraries /home/the-beast/Arduino/libraries -fqbn=archlinux-arduino:avr:uno -vid-pid=2341_0043 -ide-version=10819 -build-path /tmp/arduino_build_212870 -warnings=none -build-cache /tmp/arduino_cache_824431 -prefs=build.warn_data_percentage=75 -verbose /home/the-beast/Arduino/libraries/MFRC522/examples/DumpInfo/DumpInfo.ino
/usr/share/arduino/arduino-builder -compile -logger=machine -hardware /usr/share/arduino/hardware -tools /usr/share/arduino/tools-builder -libraries /home/the-beast/Arduino/libraries -fqbn=archlinux-arduino:avr:uno -vid-pid=2341_0043 -ide-version=10819 -build-path /tmp/arduino_build_212870 -warnings=none -build-cache /tmp/arduino_cache_824431 -prefs=build.warn_data_percentage=75 -verbose /home/the-beast/Arduino/libraries/MFRC522/examples/DumpInfo/DumpInfo.ino
Using board 'uno' from platform in folder: /usr/share/arduino/hardware/archlinux-arduino/avr
Using core 'arduino' from platform in folder: /usr/share/arduino/hardware/archlinux-arduino/avr
Detecting libraries used...
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard /tmp/arduino_build_212870/sketch/DumpInfo.ino.cpp -o /dev/null
Alternatives for SPI.h: [SPI@1.0]
ResolveLibrary(SPI.h)
  -> candidates: [SPI@1.0]
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src /tmp/arduino_build_212870/sketch/DumpInfo.ino.cpp -o /dev/null
Alternatives for MFRC522.h: [MFRC522@1.4.7]
ResolveLibrary(MFRC522.h)
  -> candidates: [MFRC522@1.4.7]
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /tmp/arduino_build_212870/sketch/DumpInfo.ino.cpp -o /dev/null
Using cached library dependencies for file: /usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src/SPI.cpp
Using cached library dependencies for file: /home/the-beast/Arduino/libraries/MFRC522/src/MFRC522.cpp
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp -o /dev/null
Generating function prototypes...
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -flto -w -x c++ -E -CC -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /tmp/arduino_build_212870/sketch/DumpInfo.ino.cpp -o /tmp/arduino_build_212870/preproc/ctags_target_for_gcc_minus_e.cpp
/usr/bin/arduino-ctags -u --language-force=c++ -f - --c++-kinds=svpf --fields=KSTtzns --line-directives /tmp/arduino_build_212870/preproc/ctags_target_for_gcc_minus_e.cpp
Compiling sketch...
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /tmp/arduino_build_212870/sketch/DumpInfo.ino.cpp -o /tmp/arduino_build_212870/sketch/DumpInfo.ino.cpp.o
Compiling libraries...
Compiling library "SPI"
Using previously compiled file: /tmp/arduino_build_212870/libraries/SPI/SPI.cpp.o
Compiling library "MFRC522"
/usr/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10819 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/usr/share/arduino/hardware/archlinux-arduino/avr/cores/arduino -I/usr/share/arduino/hardware/archlinux-arduino/avr/variants/standard -I/usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI/src -I/home/the-beast/Arduino/libraries/MFRC522/src /home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp -o /tmp/arduino_build_212870/libraries/MFRC522/MFRC522Extended.cpp.o
Using previously compiled file: /tmp/arduino_build_212870/libraries/MFRC522/MFRC522.cpp.o
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp: In member function 'MFRC522::StatusCode MFRC522Extended::TCL_Transceive(TagInfo*, byte*, byte, byte*, byte*)':
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp:824:25: error: 'backlen' was not declared in this scope; did you mean 'backLen'?
  824 |         if (backData && backlen != nullptr)) {
      |                         ^~~~~~~
      |                         backLen
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp:824:44: error: expected primary-expression before ')' token
  824 |         if (backData && backlen != nullptr)) {
      |                                            ^
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp:847:33: error: 'backlen' was not declared in this scope; did you mean 'backLen'?
  847 |                 if (backData && backlen != nullptr)) {
      |                                 ^~~~~~~
      |                                 backLen
/home/the-beast/Arduino/libraries/MFRC522/src/MFRC522Extended.cpp:847:52: error: expected primary-expression before ')' token
  847 |                 if (backData && backlen != nullptr)) {
      |                                                    ^
Using library SPI at version 1.0 in folder: /usr/share/arduino/hardware/archlinux-arduino/avr/libraries/SPI 
Using library MFRC522 at version 1.4.7 in folder: /home/the-beast/Arduino/libraries/MFRC522 
exit status 1
Error compiling for board Arduino Uno.

Huh, it's "backLen" not the "backlen" you typed!
This is what I wrote you:

if (backData && backLen) {

PS: why typing it back if the name was already there?...

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