R - time series decomposition without detection of seasonality I have a time series dataset with 200 data points. I have decomposed it using the function below:

dat2 = ts(dat1, frequency = 4)
decomposeDat = decompose(dat2, "multiplicative")


I get 4 components: trend, seasonal, cyclic and irregularity. But when I check if there is seasonality present in the dataset with frequency "4", Rstudio says that there is no seasonality for this frequency. The check is performed with the following code:

dat2 = ts(dat1, frequency = 4)
fit &lt;- tbats(dat2)
seasonal &lt;- !is.null(fit$seasonal)
seasonal


seasonal returns FALSE meaning that there is no seasonality with frequency 4.

Can someone explain, why can I decompose it into a seasonality component when no seasonality is present from the check mentioned above?
Without seeing your data it is hard to tell whether there is seasonality or not. The decompose() function will try to find seasonality using a different approach than tbats() as discussed in this post and the user comments of this blog post by the author of tbats. 

I would recommend to post your data and discuss what you are trying to achieve to get more guidance.