Properly scripting onEdit - google-apps-script

Question

I have a Google Sheets file with 3 spreadsheets. Each of the first 2 contains numerous number data entries. All I want to do is change one cell value on the third (tt) if any data is changed on a different sheet. Looking at my incomplete onEdit() script, you will see I obviously don’t know what I’m doing. How to I change the value of cell ‘a5’ on third spreadsheet (tt) for any changes made anywhere on only the first two?
function onEdit(event)
{
var aa = event.SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Input-Expenses");
var ss = event.SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Input-Income-n-Assets");
var tt = event.SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Run Program");
var r = event.aa.getActiveRange('a1:r105');
var s = event.ss.getActiveRange('d1:q94');
var g = 0
tt.getRange('a5').setValue(g);
}

Answer 1

You need to refer to this article:
https://developers.google.com/apps-script/guides/triggers/
Find Edit trigger.
range A Range object, representing the cell or range of cells that
were edited.
Usage:
var editedRange = event.range;
var editedSheet = editedRange.getSheet();
if (editedSheet.getName() === 'Sheet1')
{
var targetSheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Sheet2");
// do smth
}

Answer 2

This should do what you want
function onEdit(event) {
try {
var aName = event.range.getSheet().getName(); // Active sheet
if( ( aName === "Input-Expenses" ) || ( aName === "Input-Income-n-Assets" ) ) {
// You said anywhere on these 2 sheets and put in Run Program:A5
event.source.getSheetByName("Run Program").getRange("A5").setValue(0);
}
}
catch(err) {
SpreadsheetApp.getUi().alert(err);
}
}