The perfect answer to this kind of question does not exist since everyone learns differently (and at different speed) but I do think the best way to learn something quickly is to do simply do it.
Find a good tutorial or article explaining how to build a webapp from the ground, and reproduce the steps yourself as you read it. Once you feel comfortable with the tech/language/framework/whatever you are learning, try to break free from the tutorials and code something yourself. Having a concrete project in mind is also very helpful: you'll feel more motivated to learn something if you know how and why you will use it.
Also: set progressive goals and steps, take your time (learning quickly is possible, but learning too quickly is a bad idea), read a lot of documentation and don't be afraid to ask for help when you need to.
Also be careful of too high expectations, learning how to code can be quick, but nothing beats experience, you'll get better over time and that is something you cannot rush.
Edit 1: For clarification: even though it seems like a generic recommendation, this is actually how I learned web dev, so this is also personal advice/tips. Also, I voluntarily ignored the question focus on Javascript since I believe my advice to be valid for any language or tech. Although, if I had to give you a JS advice: you don't just "learn JS" in 2018, since today JS comes in many flavors (you might want to google about ES5 vs ES6) and frameworks are just as important as the language itself.
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By doing it.
The perfect answer to this kind of question does not exist since everyone learns differently (and at different speed) but I do think the best way to learn something quickly is to do simply do it.
Find a good tutorial or article explaining how to build a webapp from the ground, and reproduce the steps yourself as you read it. Once you feel comfortable with the tech/language/framework/whatever you are learning, try to break free from the tutorials and code something yourself. Having a concrete project in mind is also very helpful: you'll feel more motivated to learn something if you know how and why you will use it.
Also: set progressive goals and steps, take your time (learning quickly is possible, but learning too quickly is a bad idea), read a lot of documentation and don't be afraid to ask for help when you need to.
Also be careful of too high expectations, learning how to code can be quick, but nothing beats experience, you'll get better over time and that is something you cannot rush.
Edit 1: For clarification: even though it seems like a generic recommendation, this is actually how I learned web dev, so this is also personal advice/tips. Also, I voluntarily ignored the question focus on Javascript since I believe my advice to be valid for any language or tech. Although, if I had to give you a JS advice: you don't just "learn JS" in 2018, since today JS comes in many flavors (you might want to google about ES5 vs ES6) and frameworks are just as important as the language itself.