Q:

Find if a LatLng Coord is between two other LatLng Coords

Objective
To know if a coordinate point is between two other coordinates.
Background
I am making a google-maps application, and I need to know if a certain Point is between two LatLng points (start and end). 
I am looking for a function like the following:
var currentCoord = {lat: 51.8732, lng: -118.6346};
var startCoord = {lat: 61.3434, lng: -118.0046};
var endCoord = {lat: 50.5468, lng: -118.5435};

function isBetween(startCoord, endCoord, currentCoord){
    //calculations here
    return "true if currentCoord is between startCoord and endCoord, or false otherwise";
}

What I tried
To achieve this I read several questions and threads:

check coordinate exists between two points of line looked promising, but since it is only for X and Y points in a grid, and does not take into account earth curvature and other aspects, it did not work. Also, it is closed...
Check coordinate point between two points is likely what I am also looking for, but it has no answer so far and the code in the question does not work either. I would also like to avoid a chain of if statements if possible.
Check if a point projected on a line segment is not outside it was something I also checked, but unfortunately the objective is different and I could not adapt the answers to what I am looking for.
Finally, http://www.movable-type.co.uk/scripts/latlong.html is a huge library of assets for coordinates, but I fail to see how any of them could help me

Code
No matter what i try, I cannot make it work, but I do have a minimal example with my failed experiments:

"use strict";

/*global google*/

function initialize() {

  let mapOptions = {
    zoom: 3,
    center: new google.maps.LatLng(0, -180),
    mapTypeId: google.maps.MapTypeId.TERRAIN
  };

  let map = new google.maps.Map(document.getElementById('map-canvas'),
    mapOptions);

  let flightPlanCoordinates = [
    new google.maps.LatLng(37.772323, -122.214897),
    new google.maps.LatLng(21.291982, -157.821856),
    new google.maps.LatLng(-18.142599, 178.431),
    new google.maps.LatLng(-27.46758, 153.027892)
  ];

  let flightPath = new google.maps.Polyline({
    path: flightPlanCoordinates,
    geodesic: true,
    strokeColor: '#FF0000',
    strokeOpacity: 1.0,
    strokeWeight: 2
  });

  flightPath.setMap(map);

  google.maps.event.addListener(flightPath, 'mouseover', function(event) {
    console.log("Marker is over the polyline");
  });

  let marker = new google.maps.Marker({
    position: new google.maps.LatLng(37.772323, -122.214897),
    draggable: true,
    map: map,
    title: 'Drag me!'
  });

  marker.addListener('drag', function(event) {

    let startPoint = {
      lat: 37.772323,
      lng: -122.214897
    };
    let endPoint = {
      lat: 21.291982,
      lng: -157.821856
    };
    let currentPoint = {
      lat: marker.getPosition().lat(),
      lng: marker.getPosition().lng()
    };

    if (checkCoordinate(startPoint, endPoint, currentPoint))
      console.log("in line !");
  });
}

google.maps.event.addDomListener(window, 'load', initialize);

function checkCoordinate(start, end, point) {
  var slope = (end.lng - start.lng) / (end.lat - start.lat);
  var newSlope = (end.lng - point.lng) / (end.lat - point.lat);
  return (point.lat > start.lat && point.lat < end.lat && point.lng > start.lng && point.lng < end.lng && slope == newSlope);
}
html,
body,
#map-canvas {
  height: 100%;
  margin: 0px;
  padding: 0px
}
<!DOCTYPE html>
<html>

<head>
  <meta name="viewport" content="initial-scale=1.0, user-scalable=no">
  <meta charset="utf-8">
  <title>Simple Polylines</title>
  <link rel="stylesheet" type="text/css" href="style.css">
  <script src="https://maps.googleapis.com/maps/api/js?v=3"></script>
  <script src="script.js" type="text/javascript"></script>
</head>

<body>
  <div id="map-canvas"></div>
</body>

</html>

Question

How can I find out if a given coordinate is between two coordinates using a mathematical approach?

A:

One option would be to use the google.maps.geometry.poly.isLocationOnEdge method.
code snippet:

var map;

function initialize() {

  var mapOptions = {
    zoom: 2,
    center: new google.maps.LatLng(0, -180),
    mapTypeId: google.maps.MapTypeId.TERRAIN
  };

  map = new google.maps.Map(document.getElementById('map-canvas'),
    mapOptions);

  var flightPlanCoordinates = [
    new google.maps.LatLng(37.772323, -122.214897),
    new google.maps.LatLng(21.291982, -157.821856),
    new google.maps.LatLng(-18.142599, 178.431),
    new google.maps.LatLng(-27.46758, 153.027892)
  ];

  var flightPath = new google.maps.Polyline({
    path: flightPlanCoordinates,
    geodesic: false,
    strokeColor: '#FF0000',
    strokeOpacity: 1.0,
    strokeWeight: 2
  });

  flightPath.setMap(map);

  var marker = new google.maps.Marker({
    position: new google.maps.LatLng(37.772323, -122.214897),
    draggable: true,
    map: map,
    title: 'Drag me!'
  });

  marker.addListener('dragend', function(event) {

    var startPoint = {
      lat: 37.772323,
      lng: -122.214897
    };
    var startMarker = new google.maps.Marker({
      position: startPoint,
      map: map
    });
    var endPoint = {
      lat: 21.291982,
      lng: -157.821856
    };
    var endMarker = new google.maps.Marker({
      position: endPoint,
      map: map
    });
    var currentPoint = {
      lat: marker.getPosition().lat(),
      lng: marker.getPosition().lng()
    };

    if (checkCoordinate(startPoint, endPoint, marker.getPosition()))
      console.log("in line !");
  });
}

google.maps.event.addDomListener(window, 'load', initialize);

function checkCoordinate(start, end, point) {
  return google.maps.geometry.poly.isLocationOnEdge(point, new google.maps.Polyline({
    map: map,
    path: [start, end]
  }), 10e-1);
}
html,
body,
#map-canvas {
  height: 100%;
  margin: 0px;
  padding: 0px
}
<!DOCTYPE html>
<html>

<head>
  <meta name="viewport" content="initial-scale=1.0, user-scalable=no">
  <meta charset="utf-8">
  <title>Simple Polylines</title>
  <link rel="stylesheet" type="text/css" href="style.css">
  <script src="https://maps.googleapis.com/maps/api/js?v=3"></script>
  <script src="script.js" type="text/javascript"></script>
</head>

<body>
  <div id="map-canvas"></div>
</body>

</html>

