Are there any good reasons NOT to use jQuery instead of plain old JavaScript? -

I have recently detected jQuery, and I can immediately see how useful and elegant it is.

I'm curious, though - there are no reasons to use it (and just use plain old javascript)? If there is no reason, should not it be fully integrated into the Javascript language?

There is no reason to use it

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There are better libraries.

This page adds weight (and for small projects that the weight is often more expensive then the profit)

Is it completely in JavaScript Should not be included?

Most of what JQuery does is, more than that related to DOM javascript.