assert: fix internal logic judgment

doc/api/assert.md

@@ -633,13 +633,12 @@ changes:
 > functions instead.
 
 If `message` is falsy, the error message is set as the values of `actual` and
-`expected` separated by the provided `operator`. If just the two `actual` and
-`expected` arguments are provided, `operator` will default to `'!='`. If
-`message` is provided as third argument it will be used as the error message and
-the other arguments will be stored as properties on the thrown object. If
-`stackStartFn` is provided, all stack frames above that function will be
-removed from stacktrace (see [`Error.captureStackTrace`]). If no arguments are
-given, the default message `Failed` will be used.
+`expected` separated by the provided `operator`. If `operator` is falsy and no
+message is set, it will default to `'!='`. If `message` is truthy, the `operator`
+will default to `'fail'` and `message` will be used for the error message. Other
+arguments will be stored as properties on the thrown object. If `stackStartFn`
+is provided, all stack frames above that function will be removed from
+stacktrace (see [`Error.captureStackTrace`][]).
 
 ```js
 const assert = require('assert').strict;

lib/assert.js

@@ -68,6 +68,17 @@ const meta = [
 const escapeFn = (str) => meta[str.charCodeAt(0)];
 
 let warned = false;
+function showMulParamDeprecation() {
+  if (warned === false) {
+    warned = true;
+    process.emitWarning(
+      'assert.fail() with more than one argument is deprecated. ' +
+        'Please use assert.strictEqual() instead or only pass a message.',
+      'DeprecationWarning',
+      'DEP0094'
+    );
+  }
+}
 
 // The assert module provides functions that throw
 // AssertionError's when particular conditions are not met. The
@@ -100,25 +111,23 @@ function fail(actual, expected, message, operator, stackStartFn) {
     message = actual;
     actual = undefined;
   } else {
-    if (warned === false) {
-      warned = true;
-      process.emitWarning(
-        'assert.fail() with more than one argument is deprecated. ' +
-          'Please use assert.strictEqual() instead or only pass a message.',
-        'DeprecationWarning',
-        'DEP0094'
-      );
-    }
-    if (argsLen === 2)
-      operator = '!=';
+    showMulParamDeprecation();
   }
 
   if (message instanceof Error) throw message;
 
+  if (operator === undefined) {
+    if (message) {
+      operator = 'fail';
+    } else {
+      operator = '!=';
+    }
+  }
+
   const errArgs = {
     actual,
     expected,
-    operator: operator === undefined ? 'fail' : operator,
+    operator,
     stackStartFn: stackStartFn || fail,
     message
   };

test/parallel/test-assert-fail-deprecation.js

@@ -61,3 +61,16 @@ assert.throws(
   function foo() { assert.fail('first', 'second', 'message', '!==', foo); },
   (err) => !/^\s*at\sfoo\b/m.test(err.stack)
 );
+
+// Actual undefined = Error
+assert.throws(() => {
+  assert.fail(undefined);
+}, {
+  code: 'ERR_ASSERTION',
+  name: 'AssertionError',
+  message: 'Failed',
+  operator: 'fail',
+  actual: undefined,
+  expected: undefined,
+  generatedMessage: true
+});