selection.select()

selection.select(selector)

For each selected element, selects the first descendant element that matches the specified selector string. If no element matches the specified selector for the current element, the element at the current index will be null in the returned selection. (If the selector is null, every element in the returned selection will be null, resulting in an empty selection.) If the current element has associated data, this data is propagated to the corresponding selected element. If multiple elements match the selector, only the first matching element in document order is selected. For example, to select the first bold element in every paragraph:

var b = d3.selectAll("p").select("b");

If the selector is a function, it is evaluated for each selected element, in order, being passed the current datum (d), the current index (i), and the current group (nodes), with this as the current DOM element. It must return an element, or null if there is no matching element. For example, to select the previous sibling of each paragraph:

var previous = d3.selectAll("p").select(function() {
  return this.previousElementSibling;
});

Unlike selection.selectAll, selection.select does not affect grouping: it preserves the existing group structure and indexes, and propagates data (if any) to selected children. Grouping plays an important role in the data join. See Nested Selections and How Selections Work for more on this topic.