Docs: Add example of an extension function

Author: snatalenko

README.md

@@ -414,14 +414,16 @@ const mapping = {
     }
   },
 
-  // property mapping from an array
-  total: 'LINE_ITEMS.reduce((sum, { QTY, PRICE }) => sum + (QTY * PRICE), 0)'
+  // property mapping from an array,
+  // with a custom reducer function defined in `extensions`
+  total: '$sum(LINE_ITEMS, i => i.QTY * i.PRICE)'
 };
 
 // Pre-compiled function that can be executed any number of times
 const mapper = createMapper(mapping, {
   extensions: {
-    itemCatalog
+    itemCatalog,
+    $sum: (arr, cb) => arr.reduce((t, el) => t + cb(el), 0)
   }
 });
 

tests/unit/example.test.ts

@@ -74,14 +74,16 @@ describe('example', () => {
 				}
 			},
 
-			// property mapping from an array
-			total: 'LINE_ITEMS.reduce((sum, { QTY, PRICE }) => sum + (QTY * PRICE), 0)'
+			// property mapping from an array,
+			// with a custom reducer function defined in `extensions`
+			total: '$sum(LINE_ITEMS, i => i.QTY * i.PRICE)'
 		};
 
 		// Pre-compiled function that can be executed any number of times
 		const mapper = createMapper(mapping, {
 			extensions: {
-				itemCatalog
+				itemCatalog,
+				$sum: (arr, cb) => arr.reduce((t, el) => t + cb(el), 0)
 			}
 		});