say I have a line here ( | being my cursor)

    callFunction(); |// HEY THIS IS A REALLY LONG COMMENT

From there, I hit D, which is the same as d$. I want to then paste it so I have something like

    // HEY THIS IS A REALLY LONG COMMENT
    callFunction();

But I want to do that without entering insert mode.

To break a single line into two lines on a whitespace boundary, one pattern I find myself using often is moving the cursor to the whitespace and hitting 'r<return>'.

(The 'r' command replaces the character under the cursor with the next key typed, without going into insert mode.)

Oh that's a nice trick, I'll remember that one. As an avid user of J I've been wanting its inverse.

Isn't J the inverse?

how about remapping?

nnoremap <leader>d DO<esc>p