D3

Enter, Update and Exit

Ref.: https://www.d3indepth.com/enterexit

.join(
  function(enter) {
    ...
  },
  function(update) {
    ...
  },
  function(exit) {
    ...
  }
)

.join(
  function(enter) {
    return enter.append('circle');
  }
)

This is equivalent to:

.join('circle')