Table of Contents

• Assert

  • Class: assert.AssertionError

    • new assert.AssertionError(options)
  • Strict mode
  • Legacy mode
  • assert(value[, message])
  • assert.deepEqual(actual, expected[, message])

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

    • Comparison details
  • assert.doesNotReject(block[, error][, message])
  • assert.doesNotThrow(block[, error][, message])
  • assert.equal(actual, expected[, message])
  • assert.fail([message])
  • assert.fail(actual, expected[, message[, operator[, stackStartFn]]])
  • 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> The actual property on the error instance is going to contain this value. Internally used for the actual error input in case e.g., assert.strictEqual() is used.
  • expected <any> The expected property on the error instance is going to contain this value. Internally used for the expected error input in case e.g., assert.strictEqual() is used.
  • operator <string> The operator 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 string ERR_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:

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

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Input A expected to strictly deep-equal 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])#

• value <any> The input that is checked for being truthy.
• message <string> | <Error>

An alias of assert.ok().

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

• actual <any>
• expected <any>
• message <string> | <Error>

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 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

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

assert.deepEqual(obj1, obj3);
// OK

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

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])#

• actual <any>
• expected <any>
• message <string> | <Error>

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 and WeakSet comparison does not rely on their values. See below for further details.

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

// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Input A expected to strictly deep-equal input B:
// + expected - actual
//   {
// -   a: 1
// +   a: '1'
//   }

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

// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Input A expected to strictly deep-equal input B:
// + expected - actual
// - {}
// + Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Input A expected to strictly deep-equal input B:
// + expected - actual
// - 2018-04-26T00:49:08.604Z
// + Date {}

assert.deepStrictEqual(NaN, NaN);
// OK, because of the SameValue comparison

// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Input A expected to strictly deep-equal input B:
// + expected - actual
// - [Number: 1]
// + [Number: 2]

assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK

// Different zeros using the SameValue Comparison:
assert.deepStrictEqual(0, -0);
// AssertionError: Input A expected to strictly deep-equal input B:
// + expected - actual
// - 0
// + -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 });
// AssertionError [ERR_ASSERTION]: Input objects not identical:
// {
//   [Symbol()]: 1
// }

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

// Fails because weakMap3 has a property that weakMap1 does not contain:
assert.deepStrictEqual(weakMap1, weakMap3);
// AssertionError: Input A expected to strictly deep-equal input B:
// + expected - actual
//   WeakMap {
// -   [items unknown]
// +   [items unknown],
// +   unequal: true
//   }

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 <string> | <Error>

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. If the function does not return a promise, assert.doesNotReject() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

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 <string> | <Error>

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...':

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: Whoops

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

• actual <any>
• expected <any>
• message <string> | <Error>

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. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.fail([message])#

• message <string> | <Error> 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[, stackStartFn]]])#

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

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 stackStartFn 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 stackStartFn 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. The stack trace contains all frames from the error passed to ifError() including the potential new frames for ifError() itself. See below for an example.

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

// Create some random error frames.
let err;
(function errorFrame() {
  err = new Error('test error');
})();

(function ifErrorFrame() {
  assert.ifError(err);
})();
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
//     at ifErrorFrame
//     at errorFrame

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

• actual <any>
• expected <any>
• message <string> | <Error>

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

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

assert.notDeepEqual(obj1, obj4);
// OK

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])#

• actual <any>
• expected <any>
• message <string> | <Error>

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])#

• actual <any>
• expected <any>
• message <string> | <Error>

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. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

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

• actual <any>
• expected <any>
• message <string> | <Error>

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 [ERR_ASSERTION]: Identical input passed to 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])#

• value <any>
• message <string> | <Error>

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();
// AssertionError: No value argument passed to `assert.ok()`

assert.ok(false, 'it\'s false');
// AssertionError: it's false

// In the repl:
assert.ok(typeof 123 === 'string');
// AssertionError: false == true

// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(typeof 123 === 'string')

assert.ok(false);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(false)

assert.ok(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(0)

// Using `assert()` works the same:
assert(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert(0)

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

• block <Function> | <Promise>
• error <RegExp> | <Function> | <Object> | <Error>
• message <string> | <Error>

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. If the function does not return a promise, assert.rejects() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

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])#

• actual <any>
• expected <any>
• message <string> | <Error>

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 [ERR_ASSERTION]: Input A expected to strictly equal input B:
// + expected - actual
// - 1
// + 2

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

assert.strictEqual(1, '1');
// AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B:
// + expected - actual
// - 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. 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 <string> | <Error>

Expects the function block to throw an error.

If specified, error can be a Class, RegExp, a validation function, a validation object where each property will be tested for strict deep equality, or an instance of error where each property will be tested for strict deep equality including the non-enumerable message and name properties. When using an object, it is also possible to use a regular expression, when validating against a string property. See below for examples.

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

Custom validation object/error instance:

const err = new TypeError('Wrong value');
err.code = 404;
err.foo = 'bar';
err.info = {
  nested: true,
  baz: 'text'
};
err.reg = /abc/i;

assert.throws(
  () => {
    throw err;
  },
  {
    name: 'TypeError',
    message: 'Wrong value',
    info: {
      nested: true,
      baz: 'text'
    }
    // Note that only properties on the validation object will be tested for.
    // Using nested objects requires all properties to be present. Otherwise
    // the validation is going to fail.
  }
);

// Using regular expressions to validate error properties:
assert.throws(
  () => {
    throw err;
  },
  {
    // The `name` and `message` properties are strings and using regular
    // expressions on those will match against the string. If they fail, an
    // error is thrown.
    name: /^TypeError$/,
    message: /Wrong/,
    foo: 'bar',
    info: {
      nested: true,
      // It is not possible to use regular expressions for nested properties!
      baz: 'text'
    },
    // The `reg` property contains a regular expression and only if the
    // validation object contains an identical regular expression, it is going
    // to pass.
    reg: /abc/i
  }
);

// 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`.
);

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'
);

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. Using the same message as the thrown error message is going to result in an ERR_AMBIGUOUS_ARGUMENT error. 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.
// The first case will not throw as it does not match for the error message
// thrown by the input function!
assert.throws(throwingFirst, 'Second');
// In the next example the message has no benefit over the message from the
// error and since it is not clear if the user intended to actually match
// against the error message, Node.js thrown an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// Throws an error:
// TypeError [ERR_AMBIGUOUS_ARGUMENT]

// 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 an 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.