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Each programmer will have their own preference on code editor when writing codes, sometimes depending on which language they are using. Specifically to D3, there are several options available.
(3a). Bl.ock Builder
(4a). Other Local Editors
The three editors introduced above are so-called on-line editors. You can also choose off-line editors. Sublime Text and Brackets are popular for their excellence in syntax highlight, and web design friendly. I personally use Brackets.
<!DOCTYPE html> <meta charset="utf-8"> <style> </style> <div> </div> <script src="https://d3js.org/d3.v4.min.js"></script> <script> </script>
(1). codepen || jsfiddle || jsbin, https://css-tricks.com/forums/topic/codepen-jsfiddle-jsbin/.