svg`<path>`javascript动画未按预期工作

let roadmapSvg = document.getElementById("roadmap-svg");
let track = document.getElementById("track");
let body = document.getElementById("body");
let head = document.getElementById("head");

let totalLength = track.getTotalLength();
let trackPoints = [];
let getTrackBounds = () => track.getBoundingClientRect();
let scaleFactor;

body.style.strokeDasharray = totalLength;
body.style.strokeDashoffset = totalLength;

function setScaleFactor(){
  scaleFactor = roadmapSvg.getBoundingClientRect().width / roadmapSvg.viewBox.baseVal.width;
}   
setScaleFactor();

function setTrackPoints(){
  let divisions = 1000;
  let unitLength = totalLength / divisions;
  trackPoints = [];
  for(let i=0; i < divisions; i++){
    let length = unitLength * i;
    let {x,y} = track.getPointAtLength(length);
    trackPoints.push({x: x*scaleFactor, y: y*scaleFactor, length});
  }
}
setTrackPoints();


function draw(){
  let currentLength = getCurrentLength();
  body.style.strokeDashoffset = totalLength - currentLength;
  headPos = track.getPointAtLength(currentLength);
  head.setAttribute("cx", headPos.x);
  head.setAttribute("cy", headPos.y);
}

function getCurrentLength(){
  let centreY = window.innerHeight / 2;
  let trackBounds = getTrackBounds();
  let currentY = centreY - trackBounds.y;
  if(currentY < 0) return 0;
  
  // if currentY is greater that track height, that means the user has scrolled pass the track (and the whole svg) in such case the animation should be completed i.e. the head should be at the final position i.e. at totalLength 
  if(currentY > trackBounds.height) return totalLength;
  
  for(let point of trackPoints){
    if(point.y >= currentY){
      return point.length;
    }
  }
  
  // (For safety) Sometimes none of the conditions match bcoz of low precision... Such situation only occurs a point very close to total length... Thus...
  return totalLength;
}

document.addEventListener("scroll", draw);

window.addEventListener("resize", () => {
  setScaleFactor();
  setTrackPoints();
  draw();
});

body {
  background: #f1f1f1;
  margin: 0;
  padding: 0 20%;
  font-family: sans-serif;
}

#roadmap-svg{
  display: block;
  max-width: 600px;
  margin: 20px auto;
  overflow: visible;
}

#roadmap-svg #head{
  fill: red;
}

#roadmap-svg #track{
  fill: none;
  stroke-dashoffset: 3px;
  stroke: grey;
  stroke-width: 4;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}
#roadmap-svg #body{
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 5;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
  transition: stroke-dashoffset ease-in-out 0.2s;
}

.center-line{
  position: fixed;
  left: 0;
  right: 0;
  top: 50%;
  border-top: 1px solid red;
  background-color: rgba(255,255,255,0.9);
}

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<svg id="roadmap-svg" viewBox="0 0 760 300">
  <path  id="track" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" />
  <path id="body" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" />
  <circle id="head" cx="10" cy="10" r="10"/>
</svg>
<div class="center-line">Center Line</div>
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// Get the id of the <path> element and the length of <path>
var myline = document.getElementById("myline");
var length = myline.getTotalLength();
circle = document.getElementById("circle");
// The start position of the drawing
myline.style.strokeDasharray = length;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
myline.style.strokeDashoffset = length;

// Find scroll percentage on scroll (using cross-browser properties), and offset dash same amount as percentage scrolled
window.addEventListener("scroll", myFunction);

function myFunction() {
  // What % down is it?
  var scrollpercent = (document.documentElement.scrollTop * 18) / 2000;
  if (scrollpercent > 1) scrollpercent = 1;
  var draw = length * scrollpercent;

  // Reverse the drawing (when scrolling upwards)
  myline.style.strokeDashoffset = length - draw;

  //get point at length
  endPoint = myline.getPointAtLength(draw);
  circle.setAttribute("cx", endPoint.x);
  circle.setAttribute("cy", endPoint.y);
}

body {
  height: 2000px;
  background: #f1f1f1;
}

#circle {
  fill: red;
}

#mySVG {
  position: absolute;
  top: 15%;
  width: 100%;
  height: 1000px;
  
}

.st1 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: grey;
  stroke-width: 4;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}
.st0 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 5;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <path  class="st1" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <circle id="circle" cx="10" cy="10" r="10"/>
  <path id="myline" class="st0" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

// Get the id of the <path> element and the length of <path>
var myline = document.getElementById("myline");
var length = myline.getTotalLength();
circle = document.getElementById("circle");
// The start position of the drawing
myline.style.strokeDasharray = length;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
myline.style.strokeDashoffset = length;

// Find scroll percentage on scroll (using cross-browser properties), and offset dash same amount as percentage scrolled
window.addEventListener("scroll", myFunction);

function myFunction() {
  // What % down is it?
  var scrollpercent = (document.body.scrollTop + document.documentElement.scrollTop) / (document.documentElement.scrollHeight - document.documentElement.clientHeight);
  // Length to offset the dashes
  var draw = length * scrollpercent;

  // Reverse the drawing (when scrolling upwards)
  myline.style.strokeDashoffset = length - draw;

  //get point at length
  endPoint = myline.getPointAtLength(draw);
  circle.setAttribute("cx", endPoint.x);
  circle.setAttribute("cy", endPoint.y);
}

body {
  margin: 0;
  height: 1000px;
  background: #f1f1f1;
}
#circle {
  fill: red;
}
#mySVG {
  top: 15%;
  position: absolute;
  width: 100%;
}
.st1 {
  fill: none;
  stroke-dashoffset: 1;
  stroke: grey;
  stroke-width: .5;
  stroke-miterlimit: 1;
  stroke-dasharray: 2;
}
.st0 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 1;
  stroke-miterlimit: 1;
  stroke-dasharray: 2;
}
.grid {
  position: fixed;
  width: 1px;
  height: 100%;
  background: blue;
  left: 50%;
  top: 0;
}

<div class="grid"></div>
<svg id="mySVG" viewBox="0 0 200 72" preserveAspectRatio="xMidYMin slice">
  <path class="st1" stroke-dasharray="10,9" d="m 0,5 0,4 c 0,3 2,6 5,6 l 108,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -25,0 c -3,0 -6,3 -6,6 0,3 3,6 6,6 l 35,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -18,0 c -3,0 -5,3 -5,6 l 0,4" />
</svg>

<svg id="mySVG" viewBox="0 0 200 72" preserveAspectRatio="xMidYMin slice">
  <circle id="circle" cx="0" cy="3" r="2" />
  <path id="myline" class="st0" stroke-dasharray="10,9" d="m 0,5 0,4 c 0,3 2,6 5,6 l 108,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -25,0 c -3,0 -6,3 -6,6 0,3 3,6 6,6 l 35,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -18,0 c -3,0 -5,3 -5,6 l 0,4" />
</svg>

var mySVG = document.getElementById("mySVG");
// Get the id of the <path> element and the length of <path>
var myline = document.getElementById("myline");
var pathLength = myline.getTotalLength();
circle = document.getElementById("circle");
// The start position of the drawing
myline.style.strokeDasharray = pathLength;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
myline.style.strokeDashoffset = pathLength;

// throttled scroll event listener
function throttle(ms, callback) {
    var timer, lastCall=0;

    return function() {
        var now = new Date().getTime(),
            diff = now - lastCall;
        if (diff >= ms) {
            lastCall = now;
            callback();
        }
    };
}

window.addEventListener("scroll", throttle(100, myFunction));

// one initial execution
myFunction();

function myFunction() {
  var center = mySVG.createSVGPoint();
  // middle of browser viewport
  center.x = window.innerWidth / 2;
  center.y = window.innerHeight / 2;
  // transform to path coordinate system
  var matrix = myline.getScreenCTM().inverse();
  center = center.matrixTransform(matrix);

  //find nearest length on path
  var draw = getNearestLength(center);

  // Reverse the drawing (when scrolling upwards)
  myline.style.strokeDashoffset = -draw - pathLength;

  //get point at length
  endPoint = myline.getPointAtLength(draw);
  circle.style.transform = "translate(" + endPoint.x + "px, " + endPoint.y + "px)";
}

function getNearestLength(point) {
  var precision = 8,
      best,
      bestLength,
      bestDistance = Infinity;
  // linear scan for coarse approximation
  for (var scan, scanLength = 0, scanDistance; scanLength <= pathLength; scanLength += precision) {
    if ((scanDistance = distance2(scan = myline.getPointAtLength(scanLength))) < bestDistance) {
      best = scan, bestLength = scanLength, bestDistance = scanDistance;
    }
  }
  // binary search for precise estimate
  precision /= 2;
  while (precision > 0.5) {
    var before,
        after,
        beforeLength,
        afterLength,
        beforeDistance,
        afterDistance;
    if ((beforeLength = bestLength - precision) >= 0 && (beforeDistance = distance2(before = myline.getPointAtLength(beforeLength))) < bestDistance) {
      best = before, bestLength = beforeLength, bestDistance = beforeDistance;
    } else if ((afterLength = bestLength + precision) <= pathLength && (afterDistance = distance2(after = myline.getPointAtLength(afterLength))) < bestDistance) {
      best = after, bestLength = afterLength, bestDistance = afterDistance;
    } else {
      precision /= 2;
    }
  }
  return bestLength;

  function distance2(p) {
    var dx = p.x - point.x,
        dy = p.y - point.y;
    return dx * dx + dy * dy;
  }
}

body {
  height: 2000px;
  background: #f1f1f1;
}

#circle {
  fill: red;
}

#mySVG {
  position: absolute;
  top: 15%;
  width: 100%;
  height: 1000px;
  
}

.st1 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: grey;
  stroke-width: 4;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}
.st0 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 5;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
  transition: stroke-dashoffset 0.2s;
}

#circle {
   transform: translate(10px, 10px);
   transition: transform 0.2s;
   }

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <path  class="st1" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <circle id="circle" cx="0" cy="0" r="10"/>
  <path id="myline" class="st0" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>