Make mergers natively composable (within the same merge-by-template version)

index.js

@@ -22,6 +22,7 @@ const wrapPath = require("@validatem/wrap-path");
 
 // FIXME: Make sure to explain the purpose of this in the documentation (multi-step merging, like in zapdb)
 let DeleteValue = Symbol("DeleteValue");
+let Template = Symbol("Template");
 
 function wrapValidationPath(basePathSegments, lastProperty, rules) {
 	let combinedPath = basePathSegments.concat(virtualProperty(lastProperty));
@@ -102,12 +103,17 @@ function mergeValue(rule, a, b, path, options) {
 			return a;
 		}
 	} else if (typeof rule === "function") {
-		if (effectiveA === undefined) {
-			return b;
-		} else if (b === undefined) {
-			return a;
+		if (Template in rule) {
+			// This is another merge-by-template merger, the user is attempting to compose them together, so we will just apply the template directly
+			return mergeValue(rule[Template], a, b, path, options);
 		} else {
-			return rule(effectiveA, b, path, options); // FIXME: Do we want to pass the original A to the function instead?
+			if (effectiveA === undefined) {
+				return b;
+			} else if (b === undefined) {
+				return a;
+			} else {
+				return rule(effectiveA, b, path, options); // FIXME: Do we want to pass the original A to the function instead?
+			}
 		}
 	} else if (typeof rule === "object") {
 		if (Array.isArray(rule)) {
@@ -130,7 +136,7 @@ module.exports = {
 			}]
 		});
 
-		return function merge(_items, _options) {
+		let merger = function merge(_items, _options) {
 			let [ items, mergeOptions ] = validateArguments(arguments, {
 				items: [ isArray, removeNullishItems ],
 				options: {
@@ -145,6 +151,10 @@ module.exports = {
 				return mergeValue(template, merged, item, [], effectiveOptions);
 			}, items[0]);
 		};
+
+		merger[Template] = template;
+
+		return merger;
 	},
 	anyProperty: function (template) {
 		/* Used for cases where an object is used like a key->value map */