Special-case Date in deep-object-assign to ensure Dates work.

package.json

@@ -38,12 +38,12 @@
     "lint-fix": "eslint --fix --ext .js,.ts .",
     "prepublishOnly": "npm run build",
     "test": "run-s test:unit test:types:check-dts test:types:tsc test:interop",
-    "test:coverage": "BABEL_ENV=test-cov nyc mocha",
+    "test:coverage": "cross-env BABEL_ENV=test-cov nyc mocha",
     "test:interop": "node interop.js",
     "test:interop:debug": "npm run test:interop -- --fail-command \"$SHELL\"",
     "test:types:check-dts": "cd test && check-dts",
     "test:types:tsc": "tsc --noemit --project tsconfig.check.json",
-    "test:unit": "BABEL_ENV=test mocha",
+    "test:unit": "cross-env BABEL_ENV=test mocha",
     "type-check": "run-s test:types:*",
     "version": "npm run contributors:update && git add package.json",
     "contributors:update": "git-authors-cli",
@@ -100,6 +100,7 @@
     "assert": "2.1.0",
     "check-dts": "0.7.2",
     "component-emitter": "1.3.0",
+    "cross-env": "^7.0.3",
     "eslint": "8.50.0",
     "git-authors-cli": "1.0.46",
     "husky": "8.0.3",

src/deep-object-assign.ts

@@ -76,6 +76,11 @@ function deepObjectAssignNonentry(...values: readonly any[]): any {
   const a = values[0];
   const b = values[1];
 
+  if (a instanceof Date && b instanceof Date) {
+    a.setTime(b.getTime());
+    return a;
+  }
+
   for (const prop of Reflect.ownKeys(b)) {
     if (!Object.prototype.propertyIsEnumerable.call(b, prop)) {
       // Ignore nonenumerable props, Object.assign() would do the same.
@@ -108,6 +113,9 @@ function clone(a: any): any {
   if (Array.isArray(a)) {
     return a.map((value: any): any => clone(value));
   } else if (typeof a === "object" && a !== null) {
+    if (a instanceof Date) {
+      return new Date(a.getTime());
+    }
     return deepObjectAssignNonentry({}, a);
   } else {
     return a;

test/deep-object-assign.test.ts

@@ -253,4 +253,40 @@ test(deepObjectAssign, (): void => {
     foo: "bar",
     deleteFoo: "foo",
   });
+
+  const date = new Date(0);
+  const date2 = new Date(50000);
+  const date3 = new Date(150000);
+
+  // Merge single date into empty object
+  given(
+    {},
+    {
+      start: date,
+    }
+  ).expect({
+    start: date,
+  });
+
+  // Merge single date into non-empty object (no overlap)
+  given(
+    { start: date },
+    {
+      end: date2,
+    }
+  ).expect({
+    start: date,
+    end: date2,
+  });
+
+  // Merge single date into non-empty object (overlap)
+  given(
+    { start: date, end: date2 },
+    {
+      end: date3,
+    }
+  ).expect({
+    start: date,
+    end: date3,
+  });
 });