To add multiple classes with function with d3 and JavaScript, we use the attr method.

For instance, we write

d3.selectAll(".user").attr("data-name", (d, i) => {
  return `Michael #${i}`;
});

to select all elements with class user with selectAll.

And then we call attr with the attribute name we want to set for each selected element.

The callback returns the value of the data-name attribute. And i is the index of the element being iterated through to set the attribute.