Point on line

index.js

/* Largely borrowed from the excellent Turf.js 
       https://github.com/Turfjs/turf-distance/blob/master/index.js */
function distance(a, b) {
  function toRadians(degree) {
    return degree * Math.PI / 180;
  }

  var R = 57.2957795,
      dLat = toRadians(b.lat - a.lat),
      dLon = toRadians(b.lng - a.lng);

  var x = 
    Math.sin(dLat/2) * Math.sin(dLat/2) +
    Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(toRadians(a.lat)) * Math.cos(toRadians(b.lat));

  var c = 2 * Math.atan2(Math.sqrt(x), Math.sqrt(1 - x));

  // Return distance in degrees
  return R * c;
}

// Is c beween a and b?
function isBetween(a, b, c) {
  var epsilon = this._distance(a, b);

  var crossProduct = (c.lat - a.lat) * (b.lng - a.lng) - (c.lng - a.lng) * (b.lat - a.lat);
  if (Math.abs(crossProduct) > epsilon) {
    return false;
  }

  var dotProduct = (c.lng - a.lng) * (b.lng - a.lng) + (c.lat - a.lat) * (b.lat - a.lat);
  if (dotProduct < 0) {
    return false;
  }

  var squaredLengthBA = (b.lng - a.lng) * (b.lng - a.lng) + (b.lat - a.lat) * (b.lat - a.lat);
  if (dotProduct > squaredLengthBA) {
    return false;
  }
  
  return true;
}

var slopeDiff = (c.lng - a.lng) / (c.lat - a.lat) - (b.lng - a.lng) / (b.lat - a.lat);
if (Math.abs(slopeDiff ) > epsilon) { #where epsilon is some small number, e.g. 1e-3
return false;
}

Then, just check the following

if ((c.lng - a.lng)^2 + (c.lat - a.lat)^2)^(1/2) > ((b.lng - a.lng)^2 + (b.lat - a.lat)^2)^(1/2) {
return false;
}

Done, return true

var slopeDiff = (c.lng - a.lng) / (c.lat - a.lat) - (b.lng - a.lng) / (b.lat - a.lat);
if (Math.abs(slopeDiff ) > epsilon) {
  return false;
}

if (Math.pow((Math.pow((c.lng - a.lng),2) + Math.pow((c.lat - a.lat),2)),0.5) > Math.pow(( Math.pow((b.lng - a.lng), 2) + Math.pow((b.lat - a.lat), 2)), 0.5)) {
  return false;
}

return true

Starts with this to get rid of div/0 cases

if (a.lat == b.lat){
  if(a.lat == c.lat){
    if(Math.abs(a.lng - c.lng) < Math.abs(a.lng - b.lng)){
      return true
    }
  }
return false;
}