Node.js util.format('%s') breaking change bug

The documentation says that util.format('%s', obj) will call String(obj) iff obj has a user-defined .toString method, but in v12.12.0 it only calls String(obj) if either obj or obj.constructor.prototype has an own property named toString, causing inherited toString implementations to be ignored.

Submitted as nodejs/node#30333

Author: cpcallen

node/format-bug.js

@@ -0,0 +1,41 @@
+/* The documentation says that util.format('%s', obj) will call
+ * String(obj) iff obj has a user-defined .toString method, but in
+ * v12.12.0 it only calls String(obj)
+ * 
+ * Submitted as https://github.com/nodejs/node/issues/30333
+ */
+
+// ES6 subclassing:
+class A {
+  toString() {
+    return 'custom A';
+  }
+}
+
+class B extends A {
+}
+
+// ES5 subclassing:
+function C() {
+};
+
+C.prototype.toString = function() {
+  return 'custom C';
+}
+
+function D() {
+  C.call(this);
+}
+
+D.prototype = Object.create(C.prototype);
+// What if we forget to set the .constructor?
+// D.prototype.constructor = D;
+
+console.log('%s', new A());  // expected/actual: custom A
+console.log('%s', new B());  // expected: custom A / actual: B {}
+console.log('%s', new C());  // expected/actual: custom C
+console.log('%s', new D());  // expected/actual: custom C (!)
+
+// Fix forgotten .constructor:
+D.prototype.constructor = D;
+console.log('%s', new D());  // expected: custom C / actual: D {}