Fix along with negative distance was undefined

packages/turf-along/README.md

@@ -6,13 +6,17 @@
 
 Takes a [LineString][1] and returns a [Point][2] at a specified distance along the line.
 
+If [distance][3] is longer than the length of [line][4], the last coordinate of the line is returned
+If [distance][3] is negative, it will count distance along the line from end to start of the line. And if
+a negative distance overshoots the length of the line, it will return the first coordinate of the line.
+
 ### Parameters
 
-* `line` **[Feature][3]<[LineString][4]>** input line
-* `distance` **[number][5]** distance along the line
-* `options` **[Object][6]?** Optional parameters
+* `line` **[Feature][5]<[LineString][6]>** input line
+* `distance` **[number][7]** distance along the line
+* `options` **[Object][8]?** Optional parameters
 
- * `options.units` **[string][7]** can be degrees, radians, miles, or kilometers (optional, default `"kilometers"`)
+ * `options.units` **[string][9]** can be degrees, radians, miles, or kilometers (optional, default `"kilometers"`)
 
 ### Examples
 
@@ -26,23 +30,27 @@ var along = turf.along(line, 200, options);
 var addToMap = [along, line]
 ```
 
-Returns **[Feature][3]<[Point][8]>** Point `distance` `units` along the line
+Returns **[Feature][5]<[Point][10]>** Point `distance` `units` along the line
 
 [1]: https://tools.ietf.org/html/rfc7946#section-3.1.4
 
 [2]: https://tools.ietf.org/html/rfc7946#section-3.1.2
 
-[3]: https://tools.ietf.org/html/rfc7946#section-3.2
+[3]: distance
+
+[4]: line
+
+[5]: https://tools.ietf.org/html/rfc7946#section-3.2
 
-[4]: https://tools.ietf.org/html/rfc7946#section-3.1.4
+[6]: https://tools.ietf.org/html/rfc7946#section-3.1.4
 
-[5]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number
+[7]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number
 
-[6]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object
+[8]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object
 
-[7]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/String
+[9]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/String
 
-[8]: https://tools.ietf.org/html/rfc7946#section-3.1.2
+[10]: https://tools.ietf.org/html/rfc7946#section-3.1.2
 
 <!-- This file is automatically generated. Please don't edit it directly. If you find an error, edit the source file of the module in question (likely index.js or index.ts), and re-run "yarn docs" from the root of the turf project. -->
 

packages/turf-along/index.ts

@@ -2,12 +2,17 @@ import { Feature, LineString, Point } from "geojson";
 import { bearing } from "@turf/bearing";
 import { destination } from "@turf/destination";
 import { distance as measureDistance } from "@turf/distance";
-import { point, Units } from "@turf/helpers";
+import { feature, point, Units } from "@turf/helpers";
 import { getGeom } from "@turf/invariant";
+import { length } from "@turf/length";
 
 /**
  * Takes a {@link LineString} and returns a {@link Point} at a specified distance along the line.
  *
+ * If {@link distance} is longer than the length of {@link line}, the last coordinate of the line is returned
+ * If {@link distance} is negative, it will count distance along the line from end to start of the line. And if
+ * a negative distance overshoots the length of the line, it will return the first coordinate of the line.
+ *
  * @name along
  * @param {Feature<LineString>} line input line
  * @param {number} distance distance along the line
@@ -28,6 +33,10 @@ function along(
  distance: number,
  options: { units?: Units } = {}
 ): Feature<Point> {
+ if (distance < 0) {
+ const lineFeature = line.type === "Feature" ? line : feature(line);
+ distance = Math.max(0, length(lineFeature, options) + distance);
+ }
  // Get Coords
  const geom = getGeom(line);
  const coords = geom.coordinates;

packages/turf-along/package.json

@@ -68,6 +68,7 @@
  "@turf/distance": "workspace:^",
  "@turf/helpers": "workspace:^",
  "@turf/invariant": "workspace:^",
+ "@turf/length": "workspace:^",
  "tslib": "^2.6.2"
  }
 }

packages/turf-along/test.ts

@@ -19,14 +19,27 @@ test("turf-along", (t) => {
  const pt6 = along(line.geometry, 2, options);
  const pt7 = along(line, 100, options);
  const pt8 = along(line.geometry, 0, options);
- const fc = featureCollection([pt1, pt2, pt3, pt4, pt5, pt6, pt7, pt8]);
+ const pt9 = along(line, -1, options);
+ const pt10 = along(line.geometry, -1, options);
+ const fc = featureCollection([
+ pt1,
+ pt2,
+ pt3,
+ pt4,
+ pt5,
+ pt6,
+ pt7,
+ pt8,
+ pt9,
+ pt10,
+ ]);
 
  fc.features.forEach((f) => {
  t.ok(f);
  t.equal(f.type, "Feature");
  t.equal(f.geometry.type, "Point");
  });
- t.equal(fc.features.length, 8);
+ t.equal(fc.features.length, 10);
  t.equal(fc.features[7].geometry.coordinates[0], pt8.geometry.coordinates[0]);
  t.equal(fc.features[7].geometry.coordinates[1], pt8.geometry.coordinates[1]);