Arrowheads at each end of axis

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <script src="https://unpkg.com/d3-jetpack"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
const svg = d3
    .select('body')
    .append('svg')
    .attr('width', 500)
    .attr('height', 500);

// Arrowhead defs
svg.append('defs')
    .append('marker')
		.at({
  			id: 'arrowhead-right',
  			refX: 5,
  			refY: 5,
  			markerWidth: 16,
  			markerHeight: 13,
		})
    .append('path')
    .at({
        d: 'M 0 0 L 5 5 L 0 10',
        stroke: 'black',
        strokeWidth: 1,
        fill: 'none',
    });
svg.append('defs')
    .append('marker')
		.at({
  			id: 'arrowhead-left',
  			refX: 0,
  			refY: 5,
  			markerWidth: 16,
  			markerHeight: 13,
		})
    .append('path')
    .at({
        d: 'M 5 0 L 0 5 L 5 10',
        stroke: 'black',
        strokeWidth: 1,
        fill: 'none',
    });

const x = d3
    .scaleLinear()
    .range([0, 400])
    .domain([0, 10]);

const xa = svg
    .append('g')
    .attr('class', 'axis')
    .attr('transform', 'translate(50,250)')
    .call(d3.axisBottom(x)
          .tickSizeInner(0)
          .tickPadding(6)
          .tickSizeOuter(0));
    
// append the markers at the start and end of our axis path
xa.select('path').at({ markerEnd: 'url(#arrowhead-right)' });
xa.select('path').at({ markerStart: 'url(#arrowhead-left)' });

</script>
</body>

https://d3js.org/d3.v4.min.js

https://unpkg.com/d3-jetpack