While not a perfect site, http://www.w3schools.com/ is my to-go place whenever I need check something. Its content might not be all fully correct, but strangely it gets to the point very efficiently for me.
I like to use both. Sometimes MDN gives an excellent technical description of something, but W3Schools tends to give a bit more examples, and lets me play with the code.
Once I know how it works, I can look at the technical description and fill in any gaps that W3S had.
W3schools is arguably the best quick reference guide for experienced developers to quickly find an answer to a question like "what's that one function called and what's the argument order?", but due to its history of promoting bad code practices is probably not the most trusted resource for somebody just learning.
If yo use chrome, google has released an extension called personal blocklist which allows you to create just that. You may find, as I have, that blocking w3schools allows MDN to often become the first result and things get dont much faster.
