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‎doc/api/assert.md‎
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‎lib/assert.js‎
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‎test/parallel/test-assert-fail.js‎
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‎test/parallel/test-assert-fail.js‎
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@@ -194,14 +194,15 @@ If the values are not equal, an `AssertionError` is thrown with a `message`
 property set equal to the value of the `message` parameter. If the `message`
 parameter is undefined, a default error message is assigned.
 
+## assert.fail(message)
 ## assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])
 <!-- YAML
 added: v0.1.21
 -->
 * `actual` {any}
 * `expected` {any}
 * `message` {any}
-* `operator` {string}
+* `operator` {string} (default: '!=')
 * `stackStartFunction` {function} (default: `assert.fail`)
 
 Throws an `AssertionError`. If `message` is falsy, the error message is set as
@@ -221,6 +222,12 @@ assert.fail(1, 2, 'fail');
 
 assert.fail(1, 2, 'whoops', '>');
 // AssertionError: whoops
+
+assert.fail('boom');
+// AssertionError: boom
+
+assert.fail('a', 'b');
+// AssertionError: 'a' != 'b'
 ```
 
 Example use of `stackStartFunction` for truncating the exception's stacktrace:
 
@@ -77,6 +77,10 @@ function getMessage(self) {
 // display purposes.
 
 function fail(actual, expected, message, operator, stackStartFunction) {
+  if (arguments.length === 1)
+    message = actual;
+  if (arguments.length === 2)
+    operator = '!=';
   throw new assert.AssertionError({
     message: message,
     actual: actual,
 
@@ -3,6 +3,36 @@
 require('../common');
 const assert = require('assert');
 
+// no args
+assert.throws(
+  () => { assert.fail(); },
+  /^AssertionError: undefined undefined undefined$/
+);
+
+// one arg = message
+assert.throws(
+  () => { assert.fail('custom message'); },
+  /^AssertionError: custom message$/
+);
+
+// two args only, operator defaults to '!='
+assert.throws(
+  () => { assert.fail('first', 'second'); },
+  /^AssertionError: 'first' != 'second'$/
+);
+
+// three args
+assert.throws(
+  () => { assert.fail('ignored', 'ignored', 'another custom message'); },
+  /^AssertionError: another custom message$/
+);
+
+// no third arg (but a fourth arg)
+assert.throws(
+  () => { assert.fail('first', 'second', undefined, 'operator'); },
+  /^AssertionError: 'first' operator 'second'$/
+);
+
 // The stackFrameFunction should exclude the foo frame
 assert.throws(
   function foo() { assert.fail('first', 'second', 'message', '!==', foo); },