metadata
(Package: rmarkdown) :
The YAML metadata of the current R Markdown document
The object metadata stores the YAML metadata of the current R Markdown document as a list, which you may use in the R code chunks, e.g. rmarkdown::metadata$title (the title of the document), rmarkdown::metadata$author, and rmarkdown::metadata$foo (if you have a YAML field named foo), etc.
providing 
.