clip-line-string.js

var slice = require("turf-line-slice-at-intersection"),
    buffer = require("turf-buffer"),
    pip = require("point-in-polygon");

module.exports = function(feature, boundary) {

  var lineStrings = coerceLineStrings(feature),
      results = [];

  poly = buffer(coercePolygon(boundary), 50, "degrees");

  lineStrings.forEach(function(ls){

    var sliced = slice(ls, poly),
        inside = pip(ls.geometry.coordinates[0], poly.geometry.coordinates[0]);

    sliced.features.forEach(function(d,i){

      if ((inside && i % 2 === 0) || (!inside && i % 2)) {
        results.push(d.geometry.coordinates);
      }

    });

  });

  return {
    type: "MultiLineString",
    coordinates: results
  };

};

function coercePolygon(feature) {

  if (Array.isArray(feature)) {
    feature = {
      type: "Polygon",
      coordinates: [feature]
    };
  }

  return feature.type === "Feature" ? feature : { type: "Feature", properties: {}, geometry: feature };

}

function coerceLineStrings(feature) {

  var coordinates;

  if (feature.type === "Feature") {
    feature = feature.geometry;
  }

  if (feature.type === "MultiLineString") {
    coordinates = feature.coordinates;
  } else {
    coordinates = [feature.coordinates];
  }

  return coordinates.map(function(ls){
    return {
      type: "Feature",
      geometry: {
        type: "LineString",
        coordinates: ls
      }
    };
  });

}