How does 'chart' work in d3.js example code?

Question

Good day, I'm learning d3.js. I'm reading this candlestick-chart example: https://observablehq.com/@d3/candlestick-chart but its code confused me besause of its first part:

chart = {
  const svg = d3.create("svg")
      .attr("viewBox", [0, 0, width, height]);

  svg.append("g")
      .call(xAxis);

  svg.append("g")
      .call(yAxis);

  const g = svg.append("g")
      .attr("stroke-linecap", "round")
      .attr("stroke", "black")
    .selectAll("g")
    .data(data)
    .join("g")
      .attr("transform", d => `translate(${x(d.date)},0)`);

  g.append("line")
      .attr("y1", d => y(d.low))
      .attr("y2", d => y(d.high));

  g.append("line")
      .attr("y1", d => y(d.open))
      .attr("y2", d => y(d.close))
      .attr("stroke-width", x.bandwidth())
      .attr("stroke", d => d.open > d.close ? d3.schemeSet1[0]
          : d.close > d.open ? d3.schemeSet1[2]
          : d3.schemeSet1[8]);

  g.append("title")
      .text(d => `${formatDate(d.date)}
Open: ${formatValue(d.open)}
Close: ${formatValue(d.close)} (${formatChange(d.open, d.close)})
Low: ${formatValue(d.low)}
High: ${formatValue(d.high)}`);

  return svg.node();
} 

Why did this example put the codes inside chart? What does the svg.node() do in this example? Thank you!

Answer 1

That's not valid JavaScript, but an example of a "cell" in Observable's custom language (which is built on JavaScript):

https://observablehq.com/@observablehq/observables-not-javascript

You can compare it to a normal function , but as a bonus, things that depend on each other are automatically recalculated when needed:

https://observablehq.com/@observablehq/how-observable-runs

So in this case, chart is a "function" that creates a chart based on some data and other settings, and then returns the SVG element ( svg.node() ).