fix: avoid using subclassed array constructor when doing map()

Re: Automattic/mongoose#7700

lib/index.js

@@ -66,7 +66,9 @@ exports.get = function (path, o, special, map) {
       // reading a property from the array items
       var paths = parts.slice(i);
 
-      return obj.map(function (item) {
+      // Need to `concat()` to avoid `map()` calling a constructor of an array
+      // subclass
+      return [].concat(obj).map(function (item) {
         return item
           ? exports.get(paths, item, special || lookup, map)
           : map(undefined);

test/index.js

@@ -1846,6 +1846,21 @@ describe('mpath', function(){
       done();
     });
 
+    it('get() underneath subclassed array', function(done) {
+      class MyArray extends Array {}
+
+      const obj = {
+        arr: new MyArray()
+      };
+      obj.arr.push({ test: 2 });
+
+      const arr = mpath.get('arr.test', obj);
+      assert.equal(arr.constructor.name, 'Array');
+      assert.ok(!(arr instanceof MyArray));
+
+      done();
+    });
+
     it('ignores setting a nested path that doesnt exist', function(done){
       var o = doc();
       assert.doesNotThrow(function(){