How to set default option in d3.js?

@gilbert 

In d3.js, you can set a default option for a selection by using the selection.property() method. Here's an example of how you can set a default option for a dropdown menu using d3.js:

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// Set default option value
var defaultOption = "Option 1";

// Create dropdown menu
var dropdown = d3.select("#dropdown")
  .append("select");

// Create options for dropdown menu
var options = dropdown.selectAll("option")
  .data(["Option 1", "Option 2", "Option 3"])
  .enter()
  .append("option")
  .text(function(d) { return d; });

// Set default option as selected
options.property("selected", function(d) {
  return d === defaultOption;
});

In this example, we first define the default option value as "Option 1". We then create a dropdown menu with three options ("Option 1", "Option 2", "Option 3"). We use the options.property("selected", ...) method to set the default option as selected by comparing each option value to the defaultOption variable.

This will result in the "Option 1" being selected by default when the dropdown menu is rendered.