The Javascript Block
The Javascript Block is the one Block that can do whatever you need.
Access edge data
Write data to edges
Directly reference Secrets, Variables, Connections and even 3rd party client objects.
If you find that your Javascript code is regularly used then you can turn the Flow with the Javascript Block or Blocks into a Subflow.
If you find that you have a lot of Javascript Blocks working heavily with one particular system or platform then you can build a Custom Block that shields you completely from Javascript.
Editing
There are two Javascript editing modes. You can edit in place in the Flow.
When your code gets too large for in-place editing, then press the expand icon in the Block header.
Auto complete
Both edit modes have auto-complete (see the above image). With in-place editing it tends to be cut off so you're better off using the full screen editor.
Full details
Please go to the Javascript Block page for full details.