Skip to content

Commit

Permalink
doc: improve assert.markdown copy
Browse files Browse the repository at this point in the history
General improvements to assert.markdown copy including
new and improved examples

PR-URL: #4360
Reviewed-By: Trevor Norris <[email protected]>
  • Loading branch information
jasnell authored and Myles Borins committed Jan 19, 2016
1 parent 761af37 commit 823269d
Showing 1 changed file with 254 additions and 27 deletions.
281 changes: 254 additions & 27 deletions doc/api/assert.markdown
Original file line number Diff line number Diff line change
Expand Up @@ -2,42 +2,131 @@

Stability: 3 - Locked

This module is used so that Node.js can test itself. It can be accessed with
`require('assert')`. However, it is recommended that a userland assertion
library be used instead.
The `assert` module provides a simple set of assertion tests that can be used
to test invariants and implement unit tests. While the `assert` module is
generally intended for internal use by Node.js itself, it can be used by user
code calling `require('assert')`.

The API for the `assert` module is [Locked][]. This means that there will be no
additions or changes to any of the methods implemented and exposed by
the module.

## assert(value[, message]), assert.ok(value[, message])

Tests if value is truthy. It is equivalent to
Tests if `value` is truthy. It is equivalent to
`assert.equal(!!value, true, message)`.

If `value` is not truthy, 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.

const assert = require('assert');

assert(true); // OK
assert(1); // OK
assert(false);
// throws "AssertionError: false == true"
assert(0);
// throws "AssertionError: 0 == true"
assert(false, 'it\'s false');
// throws "AssertionError: it's false"

assert.ok(true); // OK
assert.ok(1); // OK
assert.ok(false);
// throws "AssertionError: false == true"
assert.ok(0);
// throws "AssertionError: 0 == true"
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"

## assert.deepEqual(actual, expected[, message])

Tests for deep equality. Primitive values are compared with the equal
comparison operator ( `==` ).
Tests for deep equality between the `actual` and `expected` parameters.
Primitive values are compared with the equal comparison operator ( `==` ).

This only considers enumerable properties. It does not test object prototypes,
attached symbols, or non-enumerable properties. This can lead to some
potentially surprising results. For example, this does not throw an
`AssertionError` because the properties on the [`Error`][] object are
non-enumerable:
Only enumerable "own" properties are considered. The `deepEqual()`
implementation does not test object prototypes, attached symbols, or
non-enumerable properties. This can lead to some potentially surprising
results. For example, the following example does not throw an `AssertionError`
because the properties on the [`Error`][] object are non-enumerable:

// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));

"Deep" equality means that the enumerable "own" properties of child objects
are evaluated also:

const assert = require('assert');

const obj1 = {
a : {
b : 1
}
};
const obj2 = {
a : {
b : 2
}
};
const obj3 = {
a : {
b : 1
}
}
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK, object is equal to itself

assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different

assert.deepEqual(obj1, obj3);
// OK, objects are equal

assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored

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.deepStrictEqual(actual, expected[, message])

Tests for deep equality. Primitive values are compared with the strict equality
operator ( `===` ).
Generally identical to `assert.deepEqual` with the exception that primitive
values are compared using the strict equality operator ( `===` ).

const assert = require('assert');

assert.deepEqual({a:1}, {a:'1'});
// OK, because 1 == '1'

assert.deepStrictEqual({a:1}, {a:'1'});
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality

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.doesNotThrow(block[, error][, message])

Expects `block` not to throw an error. See [`assert.throws()`][] for more details.
Asserts that the function `block` does not throw an error. See
[`assert.throws()`][] for more details.

When `assert.doesNotThrow()` is called, it will immediately call the `block`
function.

If an error is thrown and it is the same type as that specified by the `error`
parameter, then an `AssertionError` is thrown. If the error is of a different
type, or if the `error` parameter is undefined, the error is propagated back
to the caller.

If `block` throws an error and if it is of a different type from `error`, the
thrown error will get propagated back to the caller. The following call will
throw the [`TypeError`][], since we're not matching the error types in the
assertion.
The following, for instance, will throw the [`TypeError`][] because there is no
matching error type in the assertion:

assert.doesNotThrow(
function() {
Expand All @@ -46,8 +135,8 @@ assertion.
SyntaxError
);

In case `error` matches with the error thrown by `block`, an `AssertionError`
is thrown instead.
However, the following will result in an `AssertionError` with the message
'Got unwanted exception (TypeError)..':

assert.doesNotThrow(
function() {
Expand All @@ -56,47 +145,184 @@ is thrown instead.
TypeError
);

If an `AssertionError` is thrown and a value is provided for the `message`
parameter, the value of `message` will be appended to the `AssertionError`
message:

assert.doesNotThrow(
function() {
throw new TypeError('Wrong value');
},
TypeError,
'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops

## assert.equal(actual, expected[, message])

Tests shallow, coercive equality with the equal comparison operator ( `==` ).
Tests shallow, coercive equality between the `actual` and `expected` parameters
using the equal comparison operator ( `==` ).

const assert = require('assert');

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({a: {b: 1}}, {a: {b: 1}});
//AssertionError: { a: { b: 1 } } == { a: { b: 1 } }

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(actual, expected, message, operator)

Throws an `AssertionError`. If `message` is falsy, it displays the values for
`actual` and `expected` separated by the provided `operator`. Otherwise, it
displays `message` (and does not use `actual`, `expected`, and `operator`).
Throws an `AssertionError`. If `message` is falsy, the error message is set as
the values of `actual` and `expected` separated by the provided `operator`.
Otherwise, the error message is the value of `message`.

const assert = require('assert');

assert.fail(1, 2, undefined, '>');
// AssertionError: 1 > 2

assert.fail(1, 2, 'whoops', '>');
// AssertionError: whoops

## assert.ifError(value)

Throws `value` if `value` is truthy. This is useful when testing the `error`
argument in callbacks.

const assert = require('assert');

assert.ifError(0); // OK
assert.ifError(1); // Throws 1
assert.ifError('error') // Throws 'error'
assert.ifError(new Error()); // Throws Error

## assert.notDeepEqual(actual, expected[, message])

Tests for any deep inequality. Opposite of [`assert.deepEqual`][].

const assert = require('assert');

const obj1 = {
a : {
b : 1
}
};
const obj2 = {
a : {
b : 2
}
};
const obj3 = {
a : {
b : 1
}
}
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.deepEqual(obj1, obj2);
// OK, obj1 and obj2 are not deeply equal

assert.deepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.deepEqual(obj1, obj4);
// OK, obj1 and obj2 are not deeply equal

If the values are deeply 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.notDeepStrictEqual(actual, expected[, message])

Tests for deep inequality. Opposite of [`assert.deepStrictEqual`][].
Tests for deep strict inequality. Opposite of [`assert.deepStrictEqual`][].

const assert = require('assert');

assert.notDeepEqual({a:1}, {a:'1'});
// AssertionError: { a: 1 } notDeepEqual { a: '1' }

assert.notDeepStrictEqual({a:1}, {a:'1'});
// OK

If the values are deeply and strictly 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.notEqual(actual, expected[, message])

Tests shallow, coercive inequality with the not equal comparison operator
( `!=` ).

const assert = require('assert');

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'

If the values are 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.notStrictEqual(actual, expected[, message])

Tests strict inequality as determined by the strict not equal operator
( `!==` ).

const assert = require('assert');

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError: 1 != 1

assert.notStrictEqual(1, '1');
// OK

If the values are strictly 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.strictEqual(actual, expected[, message])

Tests strict equality as determined by the strict equality operator ( `===` ).

const assert = require('assert');

assert.strictEqual(1, 2);
// AssertionError: 1 === 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual(1, '1');
// AssertionError: 1 === '1'

If the values are not strictly 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.throws(block[, error][, message])

Expects `block` to throw an error. `error` can be a constructor, [`RegExp`][], or
validation function.
Expects the function `block` to throw an error. If specified, `error` can be a
constructor, [`RegExp`][], or validation function.

Validate instanceof using constructor:

Expand Down Expand Up @@ -130,6 +356,7 @@ Custom error validation:
'unexpected error'
);

[Locked]: documentation.html#documentation_stability_index
[`assert.deepEqual`]: #assert_assert_deepequal_actual_expected_message
[`assert.deepStrictEqual`]: #assert_assert_deepstrictequal_actual_expected_message
[`assert.throws()`]: #assert_assert_throws_block_error_message
Expand Down

0 comments on commit 823269d

Please sign in to comment.