How to create arrayformula sequence number separated by comma in google sheets

How about the following sample formula?

Sample formula:

=JOIN(",",ARRAYFORMULA(ROW(INDIRECT("A1:A"&A1))))

• When you use this formula, please put this to a cell "B1" and drag down it.
• The flow of this formula is as follows.
  1. Create a1Notation of cells using "A1:A"&A1 using INDIRECT.
  2. Retrieve row numbers from the a1Notation using ROW.
  3. Join the row numbers with , using JOIN.

Result:

Note:

When you want to put all result values using one formula, unfortunately, I couldn't find the formula using the built-in functions. In that case, I would like to propose to a custom function as follows. When you use this, please copy and paste the following script to the script editor of Spreadsheet.

And, please put a formula of =SAMPLE(A1:A) to a cell "B1". By this, the result values are put to the column "B" using the values of column "A".

const SAMPLE = v => v.map(([e]) => e && !isNaN(e) ? [...Array(e)].map((_, i) => i + 1).join(",") : "");

References:

• INDIRECT
• ROW
• JOIN
• Custom Functions in Google Sheets

You can use TEXTJOIN() with SEQUENCE() function.

=TEXTJOIN(",",TRUE,SEQUENCE(A1))

You can also use this functions in desktop Excel365