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.

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().

It can be accessed using:

const assert = require('assert').strict;

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])

• value <any>
• message <any>

An alias of assert.ok().

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

• actual <any>
• expected <any>
• message <any>

Strict mode

An alias of assert.deepStrictEqual().

Legacy mode

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, attached symbols, or non-enumerable properties — for such checks, consider using assert.deepStrictEqual() instead. This can lead to some potentially surprising results. For example, 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.

assert.deepStrictEqual(actual, expected[, message])

• actual <any>
• expected <any>
• message <any>

Generally identical to assert.deepEqual() with a few exceptions:

Comparison details

const assert = require('assert').strict;

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

// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};

Object.setPrototypeOf(fakeDate, Date.prototype);

assert.deepEqual(object, fakeDate);
// OK, doesn't check [[Prototype]]
assert.deepStrictEqual(object, fakeDate);
// AssertionError: {} deepStrictEqual Date {}
// Different [[Prototype]]

assert.deepEqual(date, fakeDate);
// OK, doesn't check type tags
assert.deepStrictEqual(date, fakeDate);
// AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {}
// Different type tags

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.

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

• block <Function>
• error <RegExp> | <Function>
• message <any>

Awaits for the promise returned by function block to complete and not be rejected. See assert.rejects() for more details.

When assert.doesNotReject() is called, it will immediately call the block function, and awaits for completion.

Besides the async nature to await the completion behaves identical to [assert.doesNotThrow()][].

(async () => {
  await assert.doesNotReject(
    async () => {
      throw new TypeError('Wrong value');
    },
    SyntaxError
  );
})();

assert.doesNotReject(
  () => Promise.reject(new TypeError('Wrong value')),
  SyntaxError
).then(() => {
  // ...
});

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

• block <Function>
• error <RegExp> | <Function>
• message <any>

Asserts that the function block does not throw an error. See assert.throws() for more details.

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.

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');
  },
  TypeError,
  'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops

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

• actual <any>
• expected <any>
• message <any>

Strict mode

An alias of assert.strictEqual().

Legacy mode

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.

assert.fail(message)

assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])

• actual <any>
• expected <any>
• message <any>
• operator <string> Default: '!='
• stackStartFunction <Function> Default: assert.fail

Throws an AssertionError. 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 only it will be used as the error message, 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).

const assert = require('assert').strict;

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

Note: In the last two cases actual, expected, and operator have no influence on the error message.

assert.fail();
// AssertionError [ERR_ASSERTION]: Failed

assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom

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

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 truthy. This is useful when testing the error argument in callbacks.

const assert = require('assert').strict;

assert.ifError(null);
// OK
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])

• actual <any>
• expected <any>
• message <any>

Strict mode

An alias of assert.notDeepStrictEqual().

Legacy mode

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.

assert.notDeepStrictEqual(actual, expected[, message])

• actual <any>
• expected <any>
• message <any>

Tests for deep strict inequality. Opposite of assert.deepStrictEqual().

const assert = require('assert').strict;

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])

• actual <any>
• expected <any>
• message <any>

Strict mode

An alias of assert.notStrictEqual().

Legacy mode

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.

assert.notStrictEqual(actual, expected[, message])

• actual <any>
• expected <any>
• message <any>

Tests strict inequality as determined by the Strict Equality Comparison ( !== ).

const assert = require('assert').strict;

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.ok(value[, message])

• value <any>
• message <any>

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').strict;

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.strictEqual(actual, expected[, message])

• actual <any>
• expected <any>
• message <any>

Tests strict equality as determined by the Strict Equality Comparison ( === ).

const assert = require('assert').strict;

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

• block <Function>
• error <RegExp> | <Function> | <Object>
• message <any>

Awaits for promise returned by function block to be rejected.

When assert.rejects() is called, it will immediately call the block function, and awaits for completion.

Besides the async nature to await the completion behaves identical to assert.throws().

If specified, error can be a constructor, RegExp, a validation function, or an object where each property will be tested for.

If specified, message will be the message provided by the AssertionError if the block fails to reject.

(async () => {
  await assert.rejects(
    async () => {
      throw new Error('Wrong value');
    },
    Error
  );
})();

assert.rejects(
  () => Promise.reject(new Error('Wrong value')),
  Error
).then(() => {
  // ...
});

assert.throws(block[, error][, message])

• block <Function>
• error <RegExp> | <Function> | <object>
• message <any>

Expects the function block to throw an error.

If specified, error can be a constructor, RegExp, a validation function, or an object where each property will be tested for.

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:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /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:

assert.throws(
  () => {
    const err = new TypeError('Wrong value');
    err.code = 404;
    throw err;
  },
  {
    name: 'TypeError',
    message: 'Wrong value'
    // Note that only properties on the error object will be tested!
  }
);

Note that error can not 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.

Caveats

For the following cases, consider using ES2015 Object.is(), which uses the SameValueZero comparison.

const a = 0;
const b = -a;
assert.notStrictEqual(a, b);
// AssertionError: 0 !== -0
// Strict Equality Comparison doesn't distinguish between -0 and +0...
assert(!Object.is(a, b));
// but Object.is() does!

const str1 = 'foo';
const str2 = 'foo';
assert.strictEqual(str1 / 1, str2 / 1);
// AssertionError: NaN === NaN
// Strict Equality Comparison can't be used to check NaN...
assert(Object.is(str1 / 1, str2 / 1));
// but Object.is() can!

For more information, see MDN's guide on equality comparisons and sameness.