In d3 version 5, you can use Promise.all
to load multiple csv files. Example:
Promise.all([
d3.csv("file1.csv"),
d3.csv("file2.csv"),
]).then(function(files) {
// files[0] will contain file1.csv
// files[1] will contain file2.csv
}).catch(function(err) {
// handle error here
})