For loop in javascript code inside rowCallback option of datatable function in R

Question

I would like to add conditional formatting to a table using datatable with rowCallback in the options. The size of the table will change in the Shiny app, and I will want to apply a background based on whether the values in the 2nd to the last column are greater than the values in the 1st column. Thus, I would like to incorporate a for loop when specifying which columns to format.

The following runs just fine when I hardcode which columns I would like to apply conditional formatting to:

if (!require(devtools)) install.packages("devtools"); library(devtools)
if (!require(DT)) devtools::install_github("rstudio/DT"); library(DT)

trial <- matrix(c(3,4,1,2,1,2,4,2,5), ncol=3)
colnames(trial) <- c('value', 'min', 'max')
trial.table <- data.frame(trial)

DT::datatable(trial.table,options = list(rowCallback = JS('
                                                          function(nRow, aData, iDisplayIndex, iDisplayIndexFull) {
                                                          if (parseFloat(aData[2]) > aData[1])
                                                          $("td:eq(2)", nRow).css("background-color", "orange");
                                                          if (parseFloat(aData[2]) > aData[1])
                                                          $("td:eq(3)", nRow).css("background-color", "orange");
                                                          }')))

However, when I try this with a loop, the display is blank:

DT::datatable(trial.table,options = list(rowCallback = JS('
                                                          function(nRow, aData, iDisplayIndex, iDisplayIndexFull) {
                                                          for (i =2, i < 4, i++) {
                                                              if (parseFloat(aData[i]) > aData[1])
                                                              $("td:eq(i)", nRow).css("background-color", "orange");
                                                              }
                                                          }')))

Do for loops work inside Javascript in R?

Answer 1

Ok, I figured this out. There are 2 issues here

1. Syntax error. The arguments in the for loop in JavaScript need to be separated by a semicolon.
2. The iterative variable i appears in double quotes in the second line of the for loop. Thus, JS doesn't know what to do with this, because it is within quotes. In order to pass the current value of i we need to bring it outside of the double quotes.

-

trial <- matrix(c(3,4,1,2,1,2,4,2,5), ncol=3)
colnames(trial) <- c('value', 'min', 'max')
trial.table <- data.frame(trial)

DT::datatable(trial.table,options = list(rowCallback = JS('
                                                          function(nRow, aData) {
                                                          for (i=2; i < 4; i++) {
                                                              if (parseFloat(aData[i]) > aData[1])
                                                              $("td:eq(" + i + ")", nRow).css("background-color", "orange");
                                                              }
                                                          }')))

Answer 2

You are missing a right-curly brace to close the for loop.