How to create elements depending on data in D3?

The simplest way is to filter your array before calling .data( ... ):

d3.select("body").selectAll("p")
    .data([4, 8, 15, 16, 23, 42].filter(function(d){ return d < 10; }))
  .enter().append("div")
    .text(function(d) { return d; });

will create a div only for 4 and 8.

Alternatively, you can filter your selection after binding your array to elements on the page to conditionally create children elements:

d3.select("body").selectAll("div")
    .data([4, 8, 15, 16, 23, 42])
  .enter().append("div")
    .text(function(d) { return d; })
 .filter(function(d){ return d == 8; }).append("span")
    .text("Equal to 8")