I tried to go for a job as a ReactJS junior dev.. I got my first interview and they liked my prototype.. but..

A week later they reply: "We decided not to go with you because we hired an expert in ReactJS".

Err.. really? You're hiring expert-level ReactJS developers for a junior position?! What the frig.
You want to know what I think? This whole "It's ok that you don't know everything, you'll learn on the job" thing is a hoax. No, the job market doesn't want novices. With every single interview, I'm met with: "but you're not an expert and we can't afford that".

This reminds me of the best advice my professor (seasoned expert in the field, real engineer with more than 20 years experience) once gave me:

"The job market doesn't have the time nor patience to mollycoddle you. When you enter it, you have to already know things to an expert degree because companies want value. They're hiring you because you have these skills and knowledge.
You have to already know what they ask before they ask it. You're required to know things by yesterday, so to speak. It is an exigent industry out there. This is why we bring you the foundations - so that you go further on your own and you can take on any problem"