I'm looking to do a filldown formula in Google Apps Script. I have a "dictionary" that contains the sheet names and the columns that need to be filled. However I am running into an issue in that I can't get the sheet I need from my key value. The key returns the right string for the sheet name, but .getsheetbyname(key) "can't read property of null".

Here is what I have:

 function filldown(){
    var sheetNames = {
        "PO Tool Import":["A","AK","AL"],
        "Live Orders Aggregated":["A"],
        "Date Lookup":["A"],
        "Containerized POs":["A","B","C","D","E","F","K","R","S","T","U","V","W","X","Y","Z","AA","AB","AC","AD","AE","AF","AG","AH","AI","AJ","AK","AL","AM","AN","AO","AP","AQ","AR","AS","AT"],
        }

for (var key in sheetNames){

    var sheetName = thisBook.getSheetByName(key).getSheetName();

    Logger.log(key);
    Logger.log(sheetNames[key]);
    Logger.log(sheetName);
    };
};

Using these returns, I will then create a formula to return the index for the columns and the length of the filldown/copy to range.

Any suggestions are appreciated.