Why is oldValue ALWAYS undefined upon entering function onEdit?

In my script, I want to compare the value that has just been entered into a cell with whatever value was previously in that cell, if any.

Per https://developers.google.com/apps-script/guides/triggers/events, oldValue should be defined if (a) the edited range is a single cell and (b) the cell did actually previously contain a value. But this assertion does not bear out in practice, at least not per my attempts: instead, the value of oldValue is ALWAYS undefined.

I selected a cell that contained the word "Rome", I then typed the word "Paris" and hit the Return key.

My onEdit function contains, for debugging purposes, the following lines of script: .

function onEdit(event) {
...
  const valueBeforeEdit = event.oldvalue;
  const valueAfterEdit = event.value;
...
  outputDebuggingMsg("valueAfterEdit = " + valueAfterEdit +
                  "\n valueBeforeEdit = " + valueBeforeEdit);
...

*outputDebuggingMsg is a function I wrote to write useful data to successive rows of my "Debugging Messages" sheet, so I can later view a long series of such data.

The trouble is, the debugging output message is always as follows:

  valueAfterEdit = Paris
  valueBeforeEdit = undefined  (I expected it to say "Rome" not "undefined")

Ths same thing happens whether I try this in Chrome [version 131.0.6778.108 (Official Build) (64-bit)] OR Edge [version 131.0.2903.86 (Official build) (64-bit)].

Error cause by typo

You have an error typo that's why the output is always undefined, change your function valueBeforeEdit = event.oldvalue to oldValue.

From this:

const valueBeforeEdit = event.oldvalue;

To this:

const valueBeforeEdit = event.oldValue;

You can read more about simple triggers and installable triggers on this link - Event Objects.