Understanding QoS in MQTT.js

[hem-bhagat-tark]

Hello. I am new to MQTT.js. My node application just publishes a message to the MQTT broker and I have another application that subscribes to the messages. I would like to know what will be the behavior if I use QoS 1 and QoS 2 while publishing messages. The MQTT documentation says that the sender will store the message until it has received the PUBACK.

Will the MQTT.js library take care of this or do I need to implement it and resend the message in case of failure?