merge: Add a first merge strategy

std/merge.js

@@ -121,6 +121,46 @@ export function deep(rules) {
   return (a, b) => objectMerge2(a, b, rules);
 }
 
+/**
+ * Merge strategy merging two values by selecting the first value.
+ *
+ * **Example**:
+ *
+ * ```js
+ * let a = {
+ *   k0: 1,
+ *   o: {
+ *     o0: 'a string',
+ *   },
+ * };
+ *
+ * let b = {
+ *   k0: 2,
+ *   k1: true,
+ *   o: {
+ *     o0: 'another string',
+ *   },
+ * };
+ *
+ * mergeFull(a, b, { o: first() });
+ * ```
+ *
+ * Will give the result:
+ *
+ * ```js
+ * {
+ *   k0: 2,
+ *   k1: true,
+ *   o: {
+ *     o0: 'a string',
+ *   },
+ * }
+ * ```
+ */
+export function first() {
+  return (a, _) => a;
+}
+
 /**
  * Merge strategy merging two values by selecting the second value.
  *

std/merge.test.js

@@ -1,5 +1,5 @@
 import {
-  mix, patch, merge, mergeFull, deep, replace, deepWithKey,
+  mix, patch, merge, mergeFull, deep, first, replace, deepWithKey,
 } from './merge';
 
 test('mix objects', () => {
@@ -200,6 +200,18 @@ test('mergeFull: pick the deep merge strategy when encountering an object as rul
   expect(result.spec.containers[1].image).toEqual('sidecar:v2');
 });
 
+test('first: basic', () => {
+  const result = mergeFull(pod, sidecarImage, {
+    spec: {
+      containers: first(),
+    },
+  });
+
+  expect(result.spec.containers.length).toEqual(2);
+  expect(result.spec.containers[0].name).toEqual('my-app');
+  expect(result.spec.containers[1].image).toEqual('sidecar:v1');
+});
+
 test('replace: basic', () => {
   const result = mergeFull(pod, sidecarImage, {
     spec: {