If value that is in a column on sheet A but doesn't exist in a column on sheet B add that value to sheet B - excel

Question

I am trying to write a script that will look in a column A on sheet1 and see if it is missing any values from column J on sheet2, and if it is missing have the value added to the bottom of the column on sheet1. I found some example code (see below), however, when I modify it to work across the two sheets I get an error.
Sub Macro1()
Dim rngA As Range, rngB As Range, MySel As Range, LastRow As Long, ws As Worksheet
Set ws = ThisWorkbook.Worksheets(1)
With ws
LastRow = .Range("A" & .Rows.Count).End(xlUp).Row
Set rngA = .Range("A1:A" & LastRow)
Set rngB = .Range("B1", .Range("B" & .Rows.Count).End(xlUp))
End With
For Each cell In rngB
If IsError(Application.Match(cell.Value, rngA, 0)) Then
If MySel Is Nothing Then
Set MySel = cell
Else
Set MySel = Union(MySel, cell)
End If
End If
Next cell
If Not MySel Is Nothing Then MySel.Copy Destination:=ws.Range("A" & LastRow + 1)
End Sub
Any help to modify this to function across sheets would be greatly appreciated. Thanks!

Answer 1

You may try the following code modification, you are getting the error due to the variable cell was not declared and ws.Range("B" & .Rows.Count).End(xlUp) is not a valid range, and you should set Range B by referring to another worksheet if you want to do so:
Sub Macro1()
Dim rngA As Range, rngB As Range, MySel As Range
Dim LastRowA As Long, LastRowB As Long
Dim ws As Worksheet
Dim cell As Range
Set ws = ThisWorkbook.Worksheets(1)
With ws
LastRowA = .Range("A" & .Rows.Count).End(xlUp).Row
LastRowB = .Range("B" & .Rows.Count).End(xlUp).Row
Set rngA = .Range("A1:A" & LastRowA)
Set rngB = .Range("B1:B" & LastRowB)
End With
For Each cell In rngB.Cells
If IsError(Application.Match(cell.Value, rngA, 0)) Then
If MySel Is Nothing Then
Set MySel = cell
Else
Set MySel = Union(MySel, cell)
End If
End If
Next cell
If Not MySel Is Nothing Then MySel.Copy Destination:=ws.Range("A" & LastRowA + 1)
End Sub
Before:
After:

Answer 2

Array approach using one-liner for Match()
Instead of looping through a data range you can execute an array Match()
to compare data values with reference values by a one-liner:
data = Application.Match(ref, data, 0)
Methodical hint
Findings return the position within the reference array, whereas all non-findings (i.e. new and therefore unique values) can be identified easily by a corresponding error entry. This is used to re-write the data array exclusively by the wanted uniques. The resulting data are eventually added in the needed size to the existing data.
Note that commonly the Match() function loops asking for single search values (1st parameter) within a reference array (2nd parameter),
e.g. via Application.Match(SingleSearchValue, reference, 0).
Side note: looping through a range by means of VBA can be time consuming for greater data set, so generally I prefer an array approach.
As OP seems to refer to two sheets with different columns A and J (instead of B),
I demonstrate a solution following this requirement.
Option Explicit
Sub AppendNewItems()
'1) get data & reference arrays via function GetDatafield()
Dim data: data = GetDatafield(Sheet1, "A") ' current data
Dim ref: ref = GetDatafield(Sheet2, "J") ' reference values
Dim NewRow As Long
NewRow = UBound(data) + 1 ' get starting row for new entries
'2) look up all of the data in the reference array and write found positions to data (one-liner)
data = Application.Match(ref, data, 0)
'Edit2: check for no or only 1 reference item ' << 2021-07-23/see comment
On Error Resume Next
Debug.Print data(1, 1)
If Err.Number <> 0 Then
Err.Clear
ReDim tmp(1 To 1, 1 To 1)
tmp(1, 1) = data(1)
data = tmp
End If
'3) take only new (=unique) elements
Dim i As Long, ii As Long
For i = 1 To UBound(data) ' loop through matches
If IsError(data(i, 1)) Then ' identify new (=not found) elements by error
ii = ii + 1 ' increment uniques counter
data(ii, 1) = ref(i, 1) ' replace error element with current reference value
End If
Next
'4) add new data to column A (not more than ii elements)
If ii Then
Sheet1.Range("A" & NewRow).Resize(ii, 1) = data
End If
End Sub
Help function GetDatafield()
Function GetDatafield(sht As Worksheet, Col As String)
Dim LastRow As Long
LastRow = sht.Range(Col & sht.Rows.Count).End(xlUp).Row
'return 1-based 2-dim datafield array
GetDatafield = sht.Range(Col & "1:" & Col & LastRow).Value2
'force single value into array ' << Edit 2021-07-22/see comment
If Not IsArray(GetDatafield) Then ' or: If LastRow = 1 Then
ReDim tmp(1 To 1, 1 To 1)
tmp(1, 1) = sht.Range(Col & "1").Value2
GetDatafield = tmp ' pass 2-dim array
End If
End Function