Doubt in implementing D3partitionR chart ,I am stuck at those two steps of code mentioned

I am going through following sample example of implementation of D3 partion chart of Antonio Gullet. In that he had taken titanic dataset as an example

I didnt understand the following steps done in pre processing of the data set,help me in understanding following steps

what does the steps meant ?

data_plot[,(var_names):=lapply(var_names,function(x){data_plot[]=paste0(x,' ',data_plot[])

source :

Thanks in Advance,

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