Smoothly close an SVG path with JavaScript

I'm working on some code where a user drops a number or points/markers and a Bezier curved SVG line is drawn through them (so a smooth line connects all the points). It's based off

https://codepen.io/francoisromain/pen/dzoZZj

svg.innerHTML = svgPath(points, bezierCommand)

An example path that I've created is:

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<svg width="100px" height="100px" viewBox="100 200 500 400">
  <path d="M 346,186.5 C 374,208.875 452,237.875 458,276 C 464,314.125 433.75,340.5 370,339 C 306.25,337.5 253,307.75 203,270 C 153,232.25 134.75,209 170,188 C 205.25,167 300.5,186.5 344,186">
</svg>

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So far it's working well till I get to closing the line. If I add Z it abruptly joins the points.

The closing needs to be (if necessary) a curve so that there's a gentle transition from the last point to the first and on to the second... i.e. the whole path becomes a "smooth" loop.

I tried adding a Bezier curve but the join ends up being a sharp bump. And I tried adding another point close to the first and then using Z but again I get a sharp bump.

How can this be done?

I modified the original function to return a pathData (according to the SVG 2 SVGPathData interface draft) the array instead of a d attribute string.
This way, we can easily manipulate and sort commands.

I've also changed the coordinate structure so we're working with point objects.

[
  { x: 344, y: 186 }
]

instead of

[
  [ 344, 186 ]
]

This pretty handy, since most native methods like getPointAtLength() or properties like points also return coordinates as objects

Extend the polyline by appending the first 2 point to the point array

     points = [
      { x: 344, y: 186 },
      { x: 458, y: 276 },
      { x: 370, y: 339 },
      { x: 203, y: 270 },
      { x: 170, y: 188 },
       //duplicated points from the beginning
      { x: 344, y: 186 },
      { x: 458, y: 276 },
       
    ];

  // append first 2 points for closed paths
  if (closed) {
    points = points.concat(points.slice(0, 2));
  }

This way the function will create a smooth curve between first/last and second vertice.

Obviously, this path has an overlapping curve segment, we need to remove. Besides, the first C and needs some adjustment.

Copy last C commands 1. control point and delete last command

We can copy the last command's 1st control point to the first C command.

  // copy last commands 1st controlpoint to first curveto
  if (closed) {
    let comLast = pathData[pathData.length - 1];
    let valuesLastC = comLast.values;
    let valuesFirstC = pathData[1].values;
    
    pathData[1] = {
      type: "C",
      values: [valuesLastC[0], valuesLastC[1], valuesFirstC.slice(2)].flat()
    };
    // delete last curveto
    pathData = pathData.slice(0, pathData.length - 1); 
  }

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let points = [
  { x: 344, y: 186 },
  { x: 458, y: 276 },
  { x: 370, y: 339 },
  { x: 203, y: 270 },
  { x: 170, y: 188 }
];


let smoothing = 0.3;
let pathData = getCurvePathData(points, smoothing, true);

// serialize pathData to d attribute string
let d = pathDataToD(pathData, 1);
path.setAttribute("d", d);


// Render the svg <path> element
function getCurvePathData(points, smoothing = 0.2, closed=true){ 
  
  // append first 2 points for closed paths
  if (closed) {
    points = points.concat(points.slice(0, 2));
  }
  
  // Properties of a line
  const line = (pointA, pointB) => {
    const lengthX = pointB.x - pointA.x;
    const lengthY = pointB.y - pointA.y;
    return {
      length: Math.sqrt(Math.pow(lengthX, 2) + Math.pow(lengthY, 2)),
      angle: Math.atan2(lengthY, lengthX)
    };
  };

  // Position of a control point
  const controlPoint = (current, previous, next, reverse) => {
    const p = previous || current;
    const n = next || current;
    const o = line(p, n);

    const angle = o.angle + (reverse ? Math.PI : 0);
    const length = o.length * smoothing;

    const x = current.x + Math.cos(angle) * length;
    const y = current.y + Math.sin(angle) * length;
    return { x, y };
  };

  let pathData = [];
  pathData.push({ type: "M", values: [points[0].x, points[0].y] });

  for (let i = 1; i < points.length; i++) {
    let point = points[i];
    const cp1 = controlPoint(points[i - 1], points[i - 2], point);
    const cp2 = controlPoint(point, points[i - 1], points[i + 1], true);
    //console.log( i, 'a', a)
    const command = {
      type: "C",
      values: [cp1.x, cp1.y, cp2.x, cp2.y, point.x, point.y]
    };

    pathData.push(command);
  }
  
  // copy last commands 1st controlpoint to first curveto
  if (closed) {
    let comLast = pathData[pathData.length - 1];
    let valuesLastC = comLast.values;
    let valuesFirstC = pathData[1].values;
    
    pathData[1] = {
      type: "C",
      values: [valuesLastC[0], valuesLastC[1], valuesFirstC.slice(2)].flat()
    };
    // delete last curveto
    pathData = pathData.slice(0, pathData.length - 1); 
  }
  
  return pathData;
};

// convert pathdata to d attribute string
function pathDataToD(pathData, decimals=3){
  let d = pathData
  .map((com) => {
    return `${com.type}${com.values.map(value=>{return +value.toFixed(decimals)}).join(" ")}`;
  })
  .join(" ");
  return d;
}

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<svg viewBox="0 0 500 500" xmlns="http://www.w3.org/2000/svg" id="svg">
  <path id="pathPoly" fill="none" stroke="green"></path>
  <path id="path" fill="none" stroke="#000"></path>
</svg>

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See original post by François Romain: Smooth a Svg path with cubic bezier curves)