Nice, it actually render the definition inline without placing it in a floating window which is the normal way this feature is implemented: https://docs.microsoft.com/en-us/visualstudio/ide/media/peek...

In gnome-shell-mode [1] we can evaluate arbitrary expression and paste the content (virtually) into the buffer. Since javascript functions evaluate to their source it makes for a poor-mans peek-definition: https://imgur.com/a/7Ot7KFy

It would maybe (sometimes) be nice to even intelligently "macro expanded" the call:

    function helper(a, b) {
      let c = a + b
      return a + c
    }
    
    function p(y) {
      let x = helper(1+y, 2)
    }
    
    --> 
    
    function p(y) {
      // inlined x = helper(1+y, 2)
      let x
      {    
        let c = (1+y) + 2
        x = (1+y) + c
      }
    }

Not the best example, but you get the idea. Expansion can be tricky when the arguments are non-trivial expression. Could fall back to assignment to a variable named after the corresponding parameter.

[1]: https://github.com/paperwm/gnome-shell-mode

VS Code has this in some of the more developed language plugins and so does eclipse (at least it does for the c++ and python plugins)