You can become a MEAN Stack developer by learning all the technologies in the MEAN Stack. Learning MEAN stack is about learning all the core technologies that are a part of the MEAN stack such as MongoDB, Angular etc. These technologies are quite intimidating to get started with. one thing that can make it easy is that they all use the same language which is JavaScript. So if you wish to learn MEAN stack you should learn each individual technologies first and then learn how to combine them to build web apps.
You can read this blog that explains what MEAN stack is.
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You can also get started with MEAN stack by learning individual technologies, like:
If you wish to learn MongoDB you can do this end to end certification course on MongoDB.
Also you can learn Angular by doing this Angular course.