R, data.table: Sum all columns whose names are stored in a vector

Question

From a data.table d such as for example

require(data.table)
d = data.table(a = 1:4, b = 11:14, c = 21:24, group = c(1,1,2,2))

Answer 1

d[, lapply(.SD, sum), by = group, .SDcols = varsToSum]

   group a  b
1:     1 3 23
2:     2 7 27