Generally, I recommend using the template engine. In script, that would be:
let body = draft.processTemplate("[[body]]")
Then just use the body variable later in your script. There’s no need to update the content of the draft to reflect the change. See related docs for details.
If you just wanting to use that value in a later action step that uses templates, there’s no need to script it, just use [[body]] in that step, and maybe [[display_title]] to get your stripped title value. More details on using templates.
Hope this helps!