Add snap ends option

src/interpolatePath.js

@@ -310,19 +310,30 @@ export function pathCommandsFromString(d) {
  *
  * @param {Object[]} aCommandsInput Array of path commands
  * @param {Object[]} bCommandsInput Array of path commands
- * @param {Function} excludeSegment a function that takes a start command object and
+ * @param {(Function|Object)} interpolateOptions
+ * @param {Function} interpolateOptions.excludeSegment a function that takes a start command object and
  *   end command object and returns true if the segment should be excluded from splitting.
+ * @param {Boolean} interpolateOptions.snapEndsToInput a boolean indicating whether end of input should
+ *   be sourced from input argument or computed. 
  * @returns {Function} Interpolation function that maps t ([0, 1]) to an array of path commands.
  */
 export function interpolatePathCommands(
   aCommandsInput,
   bCommandsInput,
-  excludeSegment
+  interpolateOptions
 ) {
   // make a copy so we don't mess with the input arrays
   let aCommands = aCommandsInput == null ? [] : aCommandsInput.slice();
   let bCommands = bCommandsInput == null ? [] : bCommandsInput.slice();
 
+  const { excludeSegment, snapEndsToInput } =
+    typeof interpolateOptions === "object"
+      ? interpolateOptions
+      : {
+          excludeSegment: interpolateOptions,
+          snapEndsToInput: false,
+        };
+
   // both input sets are empty, so we don't interpolate
   if (!aCommands.length && !bCommands.length) {
     return function nullInterpolator() {
@@ -384,7 +395,7 @@ export function interpolatePathCommands(
 
   return function pathCommandInterpolator(t) {
     // at 1 return the final value without the extensions used during interpolation
-    if (t === 1) {
+    if (t === 1 && snapEndsToInput) {
       return bCommandsInput == null ? [] : bCommandsInput;
     }
 
@@ -423,14 +434,25 @@ export function interpolatePathCommands(
  *
  * @param {String} a The `d` attribute for a path
  * @param {String} b The `d` attribute for a path
- * @param {Function} excludeSegment a function that takes a start command object and
+ * @param {(Function|Object)} interpolateOptions
+ * @param {Function} interpolateOptions.excludeSegment a function that takes a start command object and
  *   end command object and returns true if the segment should be excluded from splitting.
+ * @param {Boolean} interpolateOptions.snapEndsToInput a boolean indicating whether end of input should
+ *   be sourced from input argument or computed.
  * @returns {Function} Interpolation function that maps t ([0, 1]) to a path `d` string.
  */
-export default function interpolatePath(a, b, excludeSegment) {
+export default function interpolatePath(a, b, interpolateOptions) {
   let aCommands = pathCommandsFromString(a);
   let bCommands = pathCommandsFromString(b);
 
+  const { excludeSegment, snapEndsToInput } =
+    typeof interpolateOptions === "object"
+      ? interpolateOptions
+      : {
+          excludeSegment: interpolateOptions,
+          snapEndsToInput: false,
+        };
+
   if (!aCommands.length && !bCommands.length) {
     return function nullInterpolator() {
       return '';
@@ -445,8 +467,8 @@ export default function interpolatePath(a, b, excludeSegment) {
 
   return function pathStringInterpolator(t) {
     // at 1 return the final value without the extensions used during interpolation
-    if (t === 1) {
-      return b == null ? '' : b;
+    if (t === 1 && snapEndsToInput) {
+      return b == null ? "" : b;
     }
 
     const interpolatedCommands = commandInterpolator(t);