getLastRow() doesn't work correctly in my script - google-apps-script

Question

Ok so this doesn't work:
var range = mainList.getRange(1, 1, 50, 5).getValues();
var numRows = range.getLastRow()-1;
But this does
var numRows = mainList.getLastRow()-1;
range is CLEARLY a range? So why I am I getting a typeError?
TypeError: Cannot find function getLastRow in object DATA DATA DATA DATA DATA

Answer 1

In this situation we can tell that the variable range is actually a 2D array of data from the error, so there is no method getLastRow(). There is a range.length however.
var range = mainList.getRange(1, 1, 50, 5).getValues();
var numRows = range.getLastRow()-1;
It's actually kind of hard to see the value of your second example since we can't tell where mainList came from. Take a look at mcve
If mainList is a spreadsheet then we can go to the spreadsheet documentation here and in the example here we see that we can define the range in A1Notation by specifying the name of the sheet( sometimes called tab) as so:
var ss = SpreadsheetApp.getActiveSpreadsheet();
var range = ss.getRange("Invoices!A1:D4");
Personally I usually like to take the time to do something like this:
var spreadsheet = SpreadsheetApp.getActive(); // or SpreadsheetApp.openById('ssid'); I Google Drive the ids are the primary key to locating files not names. And you can get the id of a spreadsheet right of of the url whenever you open it up.
var sheet = spreadsheet.getSheetByName('sheetname');
var range = sheet.getRange(1,1,50,5);
var valuesArray = range.getValues();
Then you can loop through values array as shown below:
for(var i=0;i<valuesArray.length;i++){
for(var j=0;j<valuesArray[i].length;j++){
var cellvalue=valuesArray[i][j];
}
}