Empty cell reference when "Loading..."

Question

I want to do a calculation with imported data from an HTML link. The data is imported every minute. There is loading time and this ensures that the cells are empty during loading. Because my formula refers to those cells, the result is "0" and this ensures that the calculations are no longer correct. Is there a way to ensure that value is taken from the cells only after it has a value (after loading) and not when it is empty during loading?

EDITOR NOTE:

From OP comment to the first answer

Only when there is a new value -> then update a cell with the new value. When it is empty (during loading) -> do nothing and keep the old value.

Answer 1

instead of =J6 use: =IF(ISNUMBER(J6), J6, )

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example: =IF(ISNUMBER(A3), A3, )

Answer 2

Formulas using built-in methods can't keep the "old value" when a condition is meet, custom functions can't do that either. One alternative, among others, is to change a bit the way that your are using the IMPORTJSON library. Instead of using one of its custom functions, use a time-drive installable trigger to add the imported value to the spreadsheet AND to add the value of cell that depends on its result.