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Node.js v10.0.0-test72549aa9cd Documentation
Table of Contents
- Assert
- Class: assert.AssertionError
- Strict mode
- Legacy mode
- assert(value[, message])
- assert.deepEqual(actual, expected[, message])
- assert.deepStrictEqual(actual, expected[, message])
- assert.doesNotReject(block[, error][, message])
- assert.doesNotThrow(block[, error][, message])
- assert.equal(actual, expected[, message])
- assert.fail([message])
- assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])
- assert.ifError(value)
- assert.notDeepEqual(actual, expected[, message])
- assert.notDeepStrictEqual(actual, expected[, message])
- assert.notEqual(actual, expected[, message])
- assert.notStrictEqual(actual, expected[, message])
- assert.ok(value[, message])
- assert.rejects(block[, error][, message])
- assert.strictEqual(actual, expected[, message])
- assert.throws(block[, error][, message])
Assert#
The assert
module provides a simple set of assertion tests that can be used to
test invariants.
A strict
and a legacy
mode exist, while it is recommended to only use
strict mode
.
For more information about the used equality comparisons see MDN's guide on equality comparisons and sameness.
Class: assert.AssertionError#
A subclass of Error
that indicates the failure of an assertion. All errors
thrown by the assert
module will be instances of the AssertionError
class.
new assert.AssertionError(options)#
options
<Object>message
<string> If provided, the error message is going to be set to this value.actual
<any> Theactual
property on the error instance is going to contain this value. Internally used for theactual
error input in case e.g.,assert.strictEqual()
is used.expected
<any> Theexpected
property on the error instance is going to contain this value. Internally used for theexpected
error input in case e.g.,assert.strictEqual()
is used.operator
<string> Theoperator
property on the error instance is going to contain this value. Internally used to indicate what operation was used for comparison (or what assertion function triggered the error).stackStartFn
<Function> If provided, the generated stack trace is going to remove all frames up to the provided function.
A subclass of Error
that indicates the failure of an assertion.
All instances contain the built-in Error
properties (message
and name
)
and:
actual
<any> Set to the actual value in case e.g.,assert.strictEqual()
is used.expected
<any> Set to the expected value in case e.g.,assert.strictEqual()
is used.generatedMessage
<boolean> Indicates if the message was auto-generated (true
) or not.code
<string> This is always set to the stringERR_ASSERTION
to indicate that the error is actually an assertion error.operator
<string> Set to the passed in operator value.
const assert = require('assert');
// Generate an AssertionError to compare the error message later:
const { message } = new assert.AssertionError({
actual: 1,
expected: 2,
operator: 'strictEqual'
});
// Verify error output:
try {
assert.strictEqual(1, 2);
} catch (err) {
assert(err instanceof assert.AssertionError);
assert.strictEqual(err.message, message);
assert.strictEqual(err.name, 'AssertionError [ERR_ASSERTION]');
assert.strictEqual(err.actual, 1);
assert.strictEqual(err.expected, 2);
assert.strictEqual(err.code, 'ERR_ASSERTION');
assert.strictEqual(err.operator, 'strictEqual');
assert.strictEqual(err.generatedMessage, true);
}
Strict mode#
When using the strict mode
, any assert
function will use the equality used
in the strict function mode. So assert.deepEqual()
will, for example,
work the same as assert.deepStrictEqual()
.
On top of that, error messages which involve objects produce an error diff instead of displaying both objects. That is not the case for the legacy mode.
It can be accessed using:
const assert = require('assert').strict;
Example error diff (the expected
, actual
, and Lines skipped
will be on a
single row):
const assert = require('assert').strict;
assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
AssertionError [ERR_ASSERTION]: Input A expected to deepStrictEqual input B:
+ expected
- actual
... Lines skipped
[
[
...
2,
- 3
+ '3'
],
...
5
]
To deactivate the colors, use the NODE_DISABLE_COLORS
environment variable.
Please note that this will also deactivate the colors in the REPL.
Legacy mode#
When accessing assert
directly instead of using the strict
property, the
Abstract Equality Comparison will be used for any function without "strict"
in its name, such as assert.deepEqual()
.
It can be accessed using:
const assert = require('assert');
It is recommended to use the strict mode
instead as the
Abstract Equality Comparison can often have surprising results. This is
especially true for assert.deepEqual()
, where the comparison rules are
lax:
// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());
assert(value[, message])#
An alias of assert.ok()
.
assert.deepEqual(actual, expected[, message])#
Strict mode
An alias of assert.deepStrictEqual()
.
Legacy mode
assert.deepStrictEqual()
instead.Tests for deep equality between the actual
and expected
parameters.
Primitive values are compared with the Abstract Equality Comparison
( ==
).
Only enumerable "own" properties are considered. The
assert.deepEqual()
implementation does not test the
[[Prototype]]
of objects or enumerable own Symbol
properties. For such checks, consider using assert.deepStrictEqual()
instead. assert.deepEqual()
can have potentially surprising results. The
following example does not throw an AssertionError
because the properties on
the RegExp
object are not enumerable:
// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());
An exception is made for Map
and Set
. Maps and Sets have their
contained items compared too, as expected.
"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. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.deepStrictEqual(actual, expected[, message])#
Tests for deep equality between the actual
and expected
parameters.
"Deep" equality means that the enumerable "own" properties of child objects
are recursively evaluated also by the following rules.
Comparison details#
- Primitive values are compared using the SameValue Comparison, used by
Object.is()
. - Type tags of objects should be the same.
[[Prototype]]
of objects are compared using the Strict Equality Comparison.- Only enumerable "own" properties are considered.
Error
names and messages are always compared, even if these are not enumerable properties.- Enumerable own
Symbol
properties are compared as well. - Object wrappers are compared both as objects and unwrapped values.
- Object properties are compared unordered.
- Map keys and Set items are compared unordered.
- Recursion stops when both sides differ or both sides encounter a circular reference.
WeakMap
andWeakSet
comparison does not rely on their values. See below for further details.
const assert = require('assert').strict;
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using SameValue comparison
// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);
assert.deepStrictEqual(object, fakeDate);
// AssertionError: {} deepStrictEqual Date {}
// Different [[Prototype]]
assert.deepStrictEqual(date, fakeDate);
// AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {}
// Different type tags
assert.deepStrictEqual(NaN, NaN);
// OK, because of the SameValue comparison
assert.deepStrictEqual(new Number(1), new Number(2));
// Fails because the wrapped number is unwrapped and compared as well.
assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.
assert.deepStrictEqual(-0, -0);
// OK
assert.deepStrictEqual(0, -0);
// AssertionError: 0 deepStrictEqual -0
const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// Fails because symbol1 !== symbol2!
const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap([[{}, {}]]);
const weakMap3 = new WeakMap();
weakMap3.unequal = true;
assert.deepStrictEqual(weakMap1, weakMap2);
// OK, because it is impossible to compare the entries
assert.deepStrictEqual(weakMap1, weakMap3);
// Fails because weakMap3 has a property that weakMap1 does not contain!
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. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.doesNotReject(block[, error][, message])#
block
<Function> | <Promise>error
<RegExp> | <Function>message
<any>
Awaits the block
promise or, if block
is a function, immediately calls the
function and awaits the returned promise to complete. It will then check that
the promise is not rejected.
If block
is a function and it throws an error synchronously,
assert.doesNotReject()
will return a rejected Promise with that error without
checking the error handler.
Please note: Using assert.doesNotReject()
is actually not useful because there
is little benefit by catching a rejection and then rejecting it again. Instead,
consider adding a comment next to the specific code path that should not reject
and keep error messages as expressive as possible.
If specified, error
can be a Class
, RegExp
or a validation
function. See assert.throws()
for more details.
Besides the async nature to await the completion behaves identically to
assert.doesNotThrow()
.
(async () => {
await assert.doesNotReject(
async () => {
throw new TypeError('Wrong value');
},
SyntaxError
);
})();
assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
.then(() => {
// ...
});
assert.doesNotThrow(block[, error][, message])#
block
<Function>error
<RegExp> | <Function>message
<any>
Asserts that the function block
does not throw an error.
Please note: Using assert.doesNotThrow()
is actually not useful because there
is no benefit by catching an error and then rethrowing it. Instead, consider
adding a comment next to the specific code path that should not throw and keep
error messages as expressive as possible.
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 specified, error
can be a Class
, RegExp
or a validation
function. See assert.throws()
for more details.
The following, for instance, will throw the TypeError
because there is no
matching error type in the assertion:
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
SyntaxError
);
However, the following will result in an AssertionError
with the message
'Got unwanted exception (TypeError)..':
assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
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(
() => {
throw new TypeError('Wrong value');
},
/Wrong value/,
'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops
assert.equal(actual, expected[, message])#
Strict mode
An alias of assert.strictEqual()
.
Legacy mode
assert.strictEqual()
instead.Tests shallow, coercive equality between the actual
and expected
parameters
using the Abstract Equality Comparison ( ==
).
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. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.fail([message])#
message
<any> Default:'Failed'
Throws an AssertionError
with the provided error message or a default error
message. If the message
parameter is an instance of an Error
then it
will be thrown instead of the AssertionError
.
const assert = require('assert').strict;
assert.fail();
// AssertionError [ERR_ASSERTION]: Failed
assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom
assert.fail(new TypeError('need array'));
// TypeError: need array
Using assert.fail()
with more than two arguments is possible but deprecated.
See below for further details.
assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])#
actual
<any>expected
<any>message
<any>operator
<string> Default:'!='
stackStartFunction
<Function> Default:assert.fail
assert.fail([message])
or other assert 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
stackStartFunction
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.
const assert = require('assert').strict;
assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'
assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2
assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail
assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops
assert.fail(1, 2, new TypeError('need array'));
// TypeError: need array
In the last three cases actual
, expected
, and operator
have no
influence on the error message.
Example use of stackStartFunction
for truncating the exception's stacktrace:
function suppressFrame() {
assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
// at repl:1:1
// at ContextifyScript.Script.runInThisContext (vm.js:44:33)
// ...
assert.ifError(value)#
value
<any>
Throws value
if value
is not undefined
or null
. This is useful when
testing the error
argument in callbacks.
const assert = require('assert').strict;
assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error
assert.notDeepEqual(actual, expected[, message])#
Strict mode
An alias of assert.notDeepStrictEqual()
.
Legacy mode
assert.notDeepStrictEqual()
instead.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.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj2);
// OK: obj1 and obj2 are not deeply equal
assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj4);
// OK: obj1 and obj4 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. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.notDeepStrictEqual(actual, expected[, message])#
Tests for deep strict inequality. Opposite of assert.deepStrictEqual()
.
const assert = require('assert').strict;
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. If the
message
parameter is an instance of an Error
then it will be thrown
instead of the AssertionError
.
assert.notEqual(actual, expected[, message])#
Strict mode
An alias of assert.notStrictEqual()
.
Legacy mode
assert.notStrictEqual()
instead.Tests shallow, coercive inequality with the Abstract Equality Comparison
( !=
).
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. If the message
parameter is an
instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.notStrictEqual(actual, expected[, message])#
Tests strict inequality between the actual
and expected
parameters as
determined by the SameValue Comparison.
const assert = require('assert').strict;
assert.notStrictEqual(1, 2);
// OK
assert.notStrictEqual(1, 1);
// AssertionError: 1 notStrictEqual 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. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
assert.ok(value[, message])#
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. If the message
parameter is an instance of an Error
then it will be thrown instead of the
AssertionError
.
If no arguments are passed in at all message
will be set to the string:
"No value argument passed to assert.ok".
Be aware that in the repl
the error message will be different to the one
thrown in a file! See below for further details.
const assert = require('assert').strict;
assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok();
// throws:
// "AssertionError: No value argument passed to `assert.ok`.
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"
// In the repl:
assert.ok(typeof 123 === 'string');
// throws:
// "AssertionError: false == true
// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// throws:
// "AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(typeof 123 === 'string')
assert.ok(false);
// throws:
// "AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(false)
assert.ok(0);
// throws:
// "AssertionError: The expression evaluated to a falsy value:
//
// assert.ok(0)
// Using `assert()` works the same:
assert(0);
// throws:
// "AssertionError: The expression evaluated to a falsy value:
//
// assert(0)
assert.rejects(block[, error][, message])#
block
<Function> | <Promise>error
<RegExp> | <Function> | <Object> | <Error>message
<any>
Awaits the block
promise or, if block
is a function, immediately calls the
function and awaits the returned promise to complete. It will then check that
the promise is rejected.
If block
is a function and it throws an error synchronously,
assert.rejects()
will return a rejected Promise with that error without
checking the error handler.
Besides the async nature to await the completion behaves identically to
assert.throws()
.
If specified, error
can be a Class
, RegExp
, a validation function,
an object where each property will be tested for, or an instance of error where
each property will be tested for including the non-enumerable message
and
name
properties.
If specified, message
will be the message provided by the AssertionError
if
the block fails to reject.
(async () => {
await assert.rejects(
async () => {
throw new TypeError('Wrong value');
},
{
name: 'TypeError',
message: 'Wrong value'
}
);
})();
assert.rejects(
Promise.reject(new Error('Wrong value')),
Error
).then(() => {
// ...
});
Note that error
cannot be a string. If a string is provided as the second
argument, then error
is assumed to be omitted and the string will be used for
message
instead. This can lead to easy-to-miss mistakes. Please read the
example in assert.throws()
carefully if using a string as the second
argument gets considered.
assert.strictEqual(actual, expected[, message])#
Tests strict equality between the actual
and expected
parameters as
determined by the SameValue Comparison.
const assert = require('assert').strict;
assert.strictEqual(1, 2);
// AssertionError: 1 strictEqual 2
assert.strictEqual(1, 1);
// OK
assert.strictEqual(1, '1');
// AssertionError: 1 strictEqual '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. If the
message
parameter is an instance of an Error
then it will be thrown
instead of the AssertionError
.
assert.throws(block[, error][, message])#
block
<Function>error
<RegExp> | <Function> | <Object> | <Error>message
<any>
Expects the function block
to throw an error.
If specified, error
can be a Class
, RegExp
, a validation function,
an object where each property will be tested for, or an instance of error where
each property will be tested for including the non-enumerable message
and
name
properties.
If specified, message
will be the message provided by the AssertionError
if
the block fails to throw.
Validate instanceof using constructor:
assert.throws(
() => {
throw new Error('Wrong value');
},
Error
);
Validate error message using RegExp
:
Using a regular expression runs .toString
on the error object, and will
therefore also include the error name.
assert.throws(
() => {
throw new Error('Wrong value');
},
/^Error: Wrong value$/
);
Custom error validation:
assert.throws(
() => {
throw new Error('Wrong value');
},
function(err) {
if ((err instanceof Error) && /value/.test(err)) {
return true;
}
},
'unexpected error'
);
Custom error object / error instance:
const err = new TypeError('Wrong value');
err.code = 404;
assert.throws(
() => {
throw err;
},
{
name: 'TypeError',
message: 'Wrong value'
// Note that only properties on the error object will be tested!
}
);
// Fails due to the different `message` and `name` properties:
assert.throws(
() => {
const otherErr = new Error('Not found');
otherErr.code = 404;
throw otherErr;
},
err // This tests for `message`, `name` and `code`.
);
Note that error
cannot be a string. If a string is provided as the second
argument, then error
is assumed to be omitted and the string will be used for
message
instead. This can lead to easy-to-miss mistakes. Please read the
example below carefully if using a string as the second argument gets
considered:
function throwingFirst() {
throw new Error('First');
}
function throwingSecond() {
throw new Error('Second');
}
function notThrowing() {}
// The second argument is a string and the input function threw an Error.
// In that case both cases do not throw as neither is going to try to
// match for the error message thrown by the input function!
assert.throws(throwingFirst, 'Second');
assert.throws(throwingSecond, 'Second');
// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, 'Second');
// AssertionError [ERR_ASSERTION]: Missing expected exception: Second
// If it was intended to match for the error message do this instead:
assert.throws(throwingSecond, /Second$/);
// Does not throw because the error messages match.
assert.throws(throwingFirst, /Second$/);
// Throws a error:
// Error: First
// at throwingFirst (repl:2:9)
Due to the confusing notation, it is recommended not to use a string as the second argument. This might lead to difficult-to-spot errors.