Line operations: slicing, chunking and nearest points

• Pass the same units to turf.length and turf.along, since kilometers versus meters differ by a factor of 1000.
• Read properties.location and properties.index from nearestPointOnLine for mileage and segment info.
• Use positive and negative distances in turf.lineOffset to place lines on either side.

var line = turf.lineString([
  [116.391263, 39.907359],
  [116.407000, 39.899000],
  [116.430000, 39.910000]
]);

var total = turf.length(line, { units: 'kilometers' });
console.log('全长', total.toFixed(3), '公里');

[0, 0.5, 1, total].forEach(function (d) {
  var p = turf.along(line, d, { units: 'kilometers' });
  console.log(d, turf.getCoord(p));
});

var a = turf.point([116.400000, 39.903000]);
var b = turf.point([116.425000, 39.909500]);

var partByPoints = turf.lineSlice(a, b, line);
var partByKm     = turf.lineSliceAlong(line, 0.8, 2.4, { units: 'kilometers' });

console.log(partByPoints.geometry.coordinates.length);
console.log(partByKm.geometry.coordinates.length);

• along beyond the line length returns the endpoint instead of throwing, which can hide bad input.
• Points that project to the same spot make lineSlice return a zero-length degenerate line.