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))

I would like to sum all variables which names are stored in the vector varsToSum by unique values of group.

varsToSum = c("a", "b")

For the above d and varsToSum, the expected outcome is

d[,list(a = sum(a), b = sum(b)),list(group)]

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

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Answer 1

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

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