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assert: improve assert.fail() API

assert.fail() has two possible function signatures, both of which are
not intuitive. It virtually guarantees that people who try to use
assert.fail() without carefully reading the docs will end up using it
incorrectly.

This change maintains backwards compatibility with the two valid uses
(arguments 1 2 and 4 supplied but argument 3 falsy, and argument 3
supplied but arguments 1 2 and 4 all falsy) but also adds the far more
intuitive first-argument-only and first-two-arguments-only
possibilities.

assert.fail('boom');
// AssertionError: boom

assert.fail('a', 'b');
// AssertionError: 'a' != 'b'

Backport-PR-URL: #15479
PR-URL: #12293
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
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Trott authored and MylesBorins committed Apr 9, 2017
1 parent 158906b commit c21fc40d0b63809f3d4b9e044e92134931750293
Showing with 42 additions and 1 deletion.
  1. +8 −1 doc/api/assert.md
  2. +4 −0 lib/assert.js
  3. +30 −0 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:
View
@@ -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); },

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