Why Use `selectAll(null)` in D3.js for Appending Elements?

Selecting null: Why Use selectAll(null) in D3?

Background

In D3, it's common to see patterns like:

var circles = svg.selectAll(null)
    .data(data)
    .enter()
    .append("circle");

This code appends elements to the DOM, but why is selectAll(null) used? Shouldn't it be selectAll("circle") or selectAll("p") for specific elements?

The Enter Selection

Explanation of "enter" selection in D3.js:

When binding data to elements, there are three scenarios:

1. Equal number of elements and data points.
2. More elements than data points.
3. More data points than elements.

In scenario #3, data points without corresponding elements belong to the "enter" selection. Using append in an "enter" selection creates new elements for the unmatched data.

Binding to Existing Elements

The proposed snippet var circles = svg.selectAll("circle").data(data) selects and binds data to existing circles. The subsequent enter() call handles data points without matching elements.

Using selectAll(null)

While using selectAll("circle") works when no circles exist, using selectAll(null) offers a consistent guarantee that the "enter" selection always corresponds to the elements in the data array. This approach ensures that new elements are appended for all unmatched data points.

Conclusion

The example below demonstrates the usage of selectAll(null) to append paragraphs to the body:

var body = d3.select("body");
var data = ["red", "blue", "green"];
var p = body.selectAll(null)
  .data(data)
  .enter()
  .append("p")
  .text(d => "I am a " + d + " paragraph!")
  .style("color", String)

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