What you shouldn't do
You should only learn jQuery for historical reasons, if ever. If you use jQuery in your apps, nobody in their right mind will hire you for a new project. It is a sign you are out of touch with modern development. jQuery was great for its time, but there is literally no reason to use it in something new.
You should also probably not learn Bootstrap in 2020. It's showing its age and there are much more modern UI kits out there that don't have the drag Bootstrap does. Bootstrap is also a total mess and pain to develop with compared to something more modern. Bootstrap is still being used for new apps though, primarally because of very cheap overseas labor with Bootstrap, so this also is not a good thing for you if you're in a 1st world country because you won't get hired. Bootstrap has a lot of example code and tutorials, but you probably won't have much fun on a Bootstrap project because the director or CTO at that company won't really know what they're doing and be trying to make something awesome, they will be trying to make a quick buck and find you expendable.
What you should do
Probably the easiest thing for you to pick up is going to be Vue. It's about 3x easier to learn than React since you don't need to pick up JSX or learn some pretty odd and complex shenanigans out of the gate to get started.
React is still the most popular front end framework (although Vue is gaining and Vue is more popular as far as people who wish they could use it, and has more stars on github). Vue is also more logical and has the most forward-thinking libraries in 2020.
You'll also get paid a lot more if you learn Vue. I am currently a consultant/CTO/Director for more than 1 very large company. In Philadelphia, where I live, I have to pay a senior Vue developer $150k/yr and these people usually have 2 or 3 waiting offers, so I have to be extra nice to them and they can make demands like working from home. On the other hand, a senior React developer I only need to pay $90k/yr, and if they turn out to be a jerk I fire them immediately because 5 other nicer people can take their place. There are just so many React developers that there is no reason to pay them a lot or put up with their grief; they've flooded the market. Please don't join them for your own sake.
You can also do the same amount of work in less time with Vue vs React, and Vue is more extensible. However, React has more tooling, more tutorials and more examples. However, React is clearly on the way out, and is clearly the worse choice financially, so make sure you choose wisely. Take it from me, a person who learned Angular/React/Vue in that order, there is absolutely NO reason to learn more than 1 front end framework; it is a waste of your life to learn more than 1; anything else is wishful thinking. Choose the right one and stick with it till it becomes unpopular.
For UI kits, if you go with Vue, you will definitely want to use Quasar Framework. Vuetify is the next runner up, but Quasar, out of the box, can take your project to a mobile app, a desktop app, a server-side app and a browser plugin with just a few lines of config. Nothing else on Earth comes close to how quick you can develop in Quasar. Quasar is also currently the fastest growing Vue UI kit in terms of amount of development work being done, rise in popularity, and rise in community. It's been that case for about 6 months now.
I have built Quasar/Vue apps for over a dozen major companies, and manage development teams of 40+ people, and every month Quasar continues to blow my pants off with how many features they add. It's like a dream compared to Vuetify or Bootstrap.
Angular and Ember are dying and dead frameworks respectively. Don't waste your time.
A great place to learn quickly and for free is scrimba.com. I have all my developers take courses here to shore up their skills. You can literally pause and TYPE INTO the videos to test your skills. It's just mindblowing.
Also, if you enjoy UI IDEs to write your code, you should be using VSCode. The whole industry has adopted it, and frankly if you show up to an interview coding in anything else most people will just find you annoying or pompous. Microsoft took the UI dev world by storm with VSCode. It puts everything else to shame in one way or another.
Also, if you are a fan of fast feedback loops, you should be using QuokkaJS (instant object feedback right in your IDE) and WallabyJS (lets you write instant unit tests). Those two plugins, although not free, speed up my development about 15% on their own.
Also, learn keyboard shortcuts. You can triple your coding speed with just the top 20 shortcuts in your IDE.
And last but not least, learn software design patterns! This will catapult you into a great developer. The most important pattern by far is the "strategy" pattern, or, the idea of ditching "if/then" or "switch" blocks for something even better :)
Also, when you think you're ready for professional work, look me up, I might have a job for you when you do. I've hired lots of people I've met online.
Hi saidcosmin :)
What you shouldn't do
You should only learn jQuery for historical reasons, if ever. If you use jQuery in your apps, nobody in their right mind will hire you for a new project. It is a sign you are out of touch with modern development. jQuery was great for its time, but there is literally no reason to use it in something new.
You should also probably not learn Bootstrap in 2020. It's showing its age and there are much more modern UI kits out there that don't have the drag Bootstrap does. Bootstrap is also a total mess and pain to develop with compared to something more modern. Bootstrap is still being used for new apps though, primarally because of very cheap overseas labor with Bootstrap, so this also is not a good thing for you if you're in a 1st world country because you won't get hired. Bootstrap has a lot of example code and tutorials, but you probably won't have much fun on a Bootstrap project because the director or CTO at that company won't really know what they're doing and be trying to make something awesome, they will be trying to make a quick buck and find you expendable.
What you should do
Probably the easiest thing for you to pick up is going to be Vue. It's about 3x easier to learn than React since you don't need to pick up JSX or learn some pretty odd and complex shenanigans out of the gate to get started.
React is still the most popular front end framework (although Vue is gaining and Vue is more popular as far as people who wish they could use it, and has more stars on github). Vue is also more logical and has the most forward-thinking libraries in 2020.
You'll also get paid a lot more if you learn Vue. I am currently a consultant/CTO/Director for more than 1 very large company. In Philadelphia, where I live, I have to pay a senior Vue developer $150k/yr and these people usually have 2 or 3 waiting offers, so I have to be extra nice to them and they can make demands like working from home. On the other hand, a senior React developer I only need to pay $90k/yr, and if they turn out to be a jerk I fire them immediately because 5 other nicer people can take their place. There are just so many React developers that there is no reason to pay them a lot or put up with their grief; they've flooded the market. Please don't join them for your own sake.
You can also do the same amount of work in less time with Vue vs React, and Vue is more extensible. However, React has more tooling, more tutorials and more examples. However, React is clearly on the way out, and is clearly the worse choice financially, so make sure you choose wisely. Take it from me, a person who learned Angular/React/Vue in that order, there is absolutely NO reason to learn more than 1 front end framework; it is a waste of your life to learn more than 1; anything else is wishful thinking. Choose the right one and stick with it till it becomes unpopular.
For UI kits, if you go with Vue, you will definitely want to use Quasar Framework. Vuetify is the next runner up, but Quasar, out of the box, can take your project to a mobile app, a desktop app, a server-side app and a browser plugin with just a few lines of config. Nothing else on Earth comes close to how quick you can develop in Quasar. Quasar is also currently the fastest growing Vue UI kit in terms of amount of development work being done, rise in popularity, and rise in community. It's been that case for about 6 months now.
I have built Quasar/Vue apps for over a dozen major companies, and manage development teams of 40+ people, and every month Quasar continues to blow my pants off with how many features they add. It's like a dream compared to Vuetify or Bootstrap.
Angular and Ember are dying and dead frameworks respectively. Don't waste your time.
A great place to learn quickly and for free is scrimba.com. I have all my developers take courses here to shore up their skills. You can literally pause and TYPE INTO the videos to test your skills. It's just mindblowing.
Also, if you enjoy UI IDEs to write your code, you should be using VSCode. The whole industry has adopted it, and frankly if you show up to an interview coding in anything else most people will just find you annoying or pompous. Microsoft took the UI dev world by storm with VSCode. It puts everything else to shame in one way or another.
Also, if you are a fan of fast feedback loops, you should be using QuokkaJS (instant object feedback right in your IDE) and WallabyJS (lets you write instant unit tests). Those two plugins, although not free, speed up my development about 15% on their own.
Also, learn keyboard shortcuts. You can triple your coding speed with just the top 20 shortcuts in your IDE.
And last but not least, learn software design patterns! This will catapult you into a great developer. The most important pattern by far is the "strategy" pattern, or, the idea of ditching "if/then" or "switch" blocks for something even better :)
Also, when you think you're ready for professional work, look me up, I might have a job for you when you do. I've hired lots of people I've met online.
Thank you so much, Nick Steele. I really appreciate your detailed response. When I feel ready, I will definitely contact you.