How to draw arc with d3.js with only outer radius, or sector of circle?

Question

To draw arc in with d3.js we need to use the function

d3.arc()
  .innerRadius(..)
  .outerRadius(..)
  ...

this make path from A point to B and back to A. What should I use to draw simple arc from A to B? In the end this will be the line defined startAngle, endAngle and radius

Answer 1

Unfortunately, there is no native D3 generator to create an arc the way you described in your question. You can write your own function to draw a circular arc, of course, nothing forbids you.

However, if you want to stick with D3 arc generator, you have a couple of alternatives.

The first one, very easy indeed, is just passing the same value for outerRadius and innerRadius. For instance, a circular arc starting at 0 degrees and ending at 90 degrees:

var svg = d3.select("svg")
.append("g")
.attr("transform", "translate(150,75)");

var arc = d3.arc()
  .innerRadius(50)
  .outerRadius(50);
  
var sector = svg.append("path")
  .attr("fill", "none")
  .attr("stroke-width", 3)
  .attr("stroke", "darkslategray")
  .attr("d", arc({startAngle:0, endAngle:(Math.PI/2)}))

<script src="https://d3js.org/d3.v4.min.js"></script>
<svg></svg>

You may argue that "the arc is actually going and coming back in the same position, and that's not a true arc", as you stated in your question ("from A point to B and back to A"). Well, for modern browsers that makes absolutely no difference.

But if you really want this nano-micro-optimisation, you can get only the first M and A commands of the path, no matter what is the innerRadius:

var myString = arc({startAngle: 0, endAngle: (Math.PI / 2)}).split(/[A-Z]/);
sector.attr("d", "M" + myString[1] + "A" + myString[2])

Here is the demo:

var svg = d3.select("svg")
  .append("g")
  .attr("transform", "translate(150,75)");

var arc = d3.arc()
  .innerRadius(0)
  .outerRadius(50);

var myString = arc({
  startAngle: 0,
  endAngle: (Math.PI / 2)
}).split(/[A-Z]/);

var sector = svg.append("path")
  .attr("fill", "none")
  .attr("stroke-width", 3)
  .attr("stroke", "darkslategray")
  .attr("d", "M" + myString[1] + "A" + myString[2])

<script src="https://d3js.org/d3.v4.min.js"></script>
<svg></svg>