MEAN Stack is actually very easy to learn, however MEAN stack is completely based on JavaScript. So if you find JavaScript to be a difficult language it is going to be difficult. But if you can learn JavaScript is very easy to learn and develop, then you can learn MEAN Stack very easily. To learn MEAN Stack you will have to learn all the technologies from the MEAN Stack individually and then combine them to build applications.
You can read this blog that explains what MEAN stack is.
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You can 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.