R convert dataframe to JSON

The jsonlite package exists to address exactly this problem: "A practical and consistent mapping between JSON data and R objects."

Its toJSON function provides this desired result with the default options:

library(jsonlite)
x <- toJSON(res1)
cat(x)

## [{"id":1,"value":"server1"},{"id":2,"value":"server2"},
## {"id":3,"value":"server3"},{"id":4,"value":"server4"},
## {"id":5,"value":"server5"}]

How about

library(rjson)
x <- toJSON(unname(split(res1, 1:nrow(res1))))
cat(x)
# [{"id":1,"value":"server1"},{"id":2,"value":"server2"},
# {"id":3,"value":"server3"},{"id":4,"value":"server4"},
# {"id":5,"value":"server5"}]

By using split() we are essentially breaking up the large data.frame into a separate data.frame for each row. And by removing the names from the resulting list, the toJSON function wraps the results in an array rather than a named object.

You can also use library(jsonify)

jsonify::to_json( res1 )
# [{"id":1.0,"value":"server1"},{"id":2.0,"value":"server2"},{"id":3.0,"value":"server3"},{"id":4.0,"value":"server4"},{"id":5.0,"value":"server5"}]

Now you can easily just call jsonlite::write_json() directly on the dataframe.