fix: ensure nothing mutates the exported 'message'

refs #sinonjs/sinon#2502

lib/create-matcher.js

@@ -60,6 +60,13 @@ function createMatcher(expectation, message) {
         m.message = `match(${valueToString(expectation)})`;
     }
 
+    // ensure that nothing mutates the exported message value, ref https://github.com/sinonjs/sinon/issues/2502
+    Object.defineProperty(m, "message", {
+        configurable: false,
+        writable: false,
+        value: m.message,
+    });
+
     return m;
 }
 

lib/create-matcher.test.js

@@ -417,6 +417,15 @@ describe("matcher", function () {
         assert(match.test("testing"));
     });
 
+    it('ensures the "message" property is unconfigurable', function () {
+        var match = createMatcher(() => true, "something");
+        assert.equals(match.message, "something");
+        assert.exception(() => {
+            match.message = "something else";
+        });
+        assert.equals(match.message, "something");
+    });
+
     describe(".toString", function () {
         it("returns message", function () {
             var message = "hello sinonMatch";