Concatenate column into single sell separated by commas

I have a large column of data that needs to concatenate and separate each cell value by a comma. Any easy way to do this?

Let's say the range were only A1:A20. (I know it's longer, but let's just say.)

1) In B20 put the formula, just duplicates the A20 value:

=A20

2) Then above that in B19, put this:

=A19&","&B20

...and copy that cell all the way UP to the top of the data. The last cell at the top will have the whole string. Copy and paste that value elsewhere.

You get the idea, now repeat that process but start at the actual bottom of your data.