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5.1.5 • Public • Published


Extensible BDD assertion toolkit inspired by expect.js

expect(window.r, 'to be', undefined);
expect({ a: 'b' }, 'to equal', { a: 'b' });
expect(5, 'to be a', 'number');
expect([], 'to be an', 'array');
expect(window, 'not to be an', Image);

NPM version Build Status Coverage Status Dependency Status

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  • Extensible
  • Fast
  • Provides really nice error messages
  • Helps you if you misspells assertions
  • Compatible with all test frameworks.
  • Node.JS ready (require('unexpected')).
  • Single global with no prototype extensions or shims.
  • Cross-browser: works on Chrome, Firefox, Safari, Opera, IE6+, (IE6-IE8 with es5-shim).

How to use


Install it with NPM or add it to your package.json:

$ npm install unexpected


var expect = require('unexpected');


Include unexpected.js.

<script src="unexpected.js"></script>

this will expose the expect function under the following namespace:

var expect = weknowhow.expect;


Include the library with RequireJS the following way:

    paths: {
        unexpected: 'path/to/unexpected'
define(['unexpected'], function (expect) {
   // Your code


to be ok

asserts that the value is truthy

ok / truthy / falsy: asserts that the value is truthy or not

expect(1, 'to be ok');
expect(true, 'to be ok');
expect(true, 'not to be falsy');
expect({}, 'to be truthy');
expect(0, 'not to be ok');
expect(0, 'to be falsy');
expect(null, 'to be falsy');
expect(undefined, 'to be falsy');

be: asserts === equality

expect(obj, 'to be', obj);
expect(obj, 'not to be', {});
expect(1, 'to be', 1);
expect(1, 'not to be', true);
expect('1', 'not to be', 1);
expect(null, 'not to be', undefined);
expect(null, 'to be null');
expect(0, 'not to be null');
expect(undefined, 'not to be null');
expect(true, 'to be true');
expect(false, 'not to be true');
expect(false, 'to be false');
expect(true, 'not to be false');
expect(undefined, 'to be undefined');
expect(null, 'to be defined');
expect(false, 'to be defined');
expect({}, 'to be defined');

equal: asserts deep equality that works with objects

expect({ a: 'b' }, 'to equal', { a: 'b' });
expect(1, 'not to equal', '1');
expect({ one: 1 }, 'not to equal', { one: '1' });
expect(null, 'not to equal', '1');
var now = new Date();
expect(now, 'to equal', now);
expect(now, 'to equal', new Date(now.getTime()));
expect({ now: now }, 'to equal', { now: now });

canonical: asserts that an object has its properties defined in sorted order at all levels

expect({ a: 123, b: 456 }, 'to be canonical');
expect([456, { a: 123 }], 'to be canonical');

a / an: asserts typeof with support for array type and instanceof

expect(5, 'to be a', 'number');
expect(5, 'to be a number');
expect('abc', 'to be a', 'string');
expect('abc', 'to be a string');
expect('', 'to be an empty string');
expect('abc', 'to be a non-empty string');
expect([], 'to be an', 'array');
expect([], 'to be an array');
expect([], 'to be an', Array);
expect([], 'to be an empty array');
expect([123], 'to be a non-empty array');
expect({foo: 123}, 'to be an', 'object');
expect({foo: 123}, 'to be an object');
expect({foo: 123}, 'to be a non-empty object');
expect({}, 'to be an empty object');
expect(null, 'not to be an', 'object');
expect(null, 'not to be an object');
expect(true, 'to be a', 'boolean');
expect(true, 'to be a boolean');
expect(expect, 'to be a', 'function');
expect(expect, 'to be a function');

NaN: asserts that the value is NaN

expect(NaN, 'to be NaN');
expect(2, 'not to be NaN');

close to: asserts that the difference between two numbers is <= epsilon

expect(1.5, 'to be close to', 1.500001, 1e-5);
expect(1.5, 'not to be close to', 1.499, 1e-4);

epsilon defaults to 1e-9 if omitted:

expect(1.5, 'to be close to', 1.5000000001);

match: asserts String regular expression match

expect('test', 'to match', /.*st/);
expect('test', 'not to match', /foo/);
expect(null, 'not to match', /foo/);

contain: asserts indexOf for an array or string

expect([1, 2], 'to contain', 1);
expect('hello world', 'to contain', 'world');

length: asserts array .length

expect([], 'to have length', 0);
expect([1,2,3], 'to have length', 3);
expect([1,2,3], 'not to have length', 4);

empty: asserts that an array or array-like object (identified by the presence of a length property) is empty

expect([], 'to be empty');
expect('', 'to be empty');
expect({ my: 'object' }, 'not to be empty');
expect([1,2,3], 'not to be empty');
expect({ length: 0, duck: 'typing' }, 'to be empty');

property: asserts presence of an own property (and value optionally)

expect([1, 2], 'to have property', 'length');
expect([1, 2], 'to have property', 'length', 2);
expect({a: 'b'}, 'to have property', 'a');
expect({a: 'b'}, 'to have property', 'a', 'b');
expect({a: 'b'}, 'to have property', 'toString');
expect({a: 'b'}, 'to have own property', 'a');
expect(Object.create({a: 'b'}), 'not to have own property', 'a');

properties: assert presence of properties in an object (and value optionally)

expect({ a: 'a', b: { c: 'c' }, d: 'd' }, 'to have properties', ['a', 'b']);
expect({ a: 'a', b: { c: 'c' }, d: 'd' }, 'to have own properties', ['a', 'b']);
expect({ a: 'a', b: { c: 'c' }, d: 'd' }, 'not to have properties', ['k', 'l']);
expect({ a: 'a', b: { c: 'c' }, d: 'd' }, 'to have properties', {
    a: 'a',
    b: { c: 'c' }
expect([ 'a', { c: 'c' }, 'd' ], 'to have properties', {
    1: { c: 'c' }
    2: 'd',

key / keys: asserts the presence of a key. Supports the only modifier

expect({ a: 'b' }, 'to have key', 'a');
expect({ a: 'b' }, 'not to have key', 'b');
expect({ a: 'b', c: 'd' }, 'to not only have key', 'a');
expect({ a: 'b', c: 'd' }, 'to only have keys', 'a', 'c');
expect({ a: 'b', c: 'd' }, 'to only have keys', ['a', 'c']);
expect({ a: 'b', c: 'd', e: 'f' }, 'to not only have keys', ['a', 'c']);

throw exception / throw error / throw: asserts that the Function throws or not when called

expect(fn, 'to throw exception');
expect(fn, 'to throw');
expect(fn, 'to throw exception', function (e) { // get the exception object
  expect(e, 'to be a', SyntaxError);
expect(fn, 'to throw exception', /matches the exception message/);
expect(fn, 'to throw error', 'matches the exact exception message');
expect(fn2, 'not to throw error');

arity: asserts that the Function takes the given number of arguments

expect(Math.max, 'to have arity', 2);

finite/infinite: asserts a finite or infinite number

expect(123, 'to be finite');
expect(Infinity, 'not to be finite');
expect(Infinity, 'to be infinite');

within: asserts a number within a range

expect(0, 'to be within', 0, 4);
expect(1, 'to be within', 0, 4);
expect(4, 'to be within', 0, 4);
expect(-1, 'not to be within', 0, 4);
expect(5, 'not to be within', 0, 4);

greater than / above: asserts >

expect(3, 'to be greater than', 2);
expect(1, 'to be above', 0);
expect(4, 'to be >', 3);
expect(4, '>', 3);

greater than or equal to: asserts >

expect(3, 'to be greater than or equal to', 2);
expect(3, 'to be >=', 3);
expect(3, '>=', 3);

less than / below: asserts <

expect(0, 'to be less than', 4);
expect(0, 'to be below', 1);
expect(3, 'to be <', 4);
expect(3, '<', 4);

less than or equal to: asserts >

expect(0, 'to be less than or equal to', 4);
expect(4, 'to be <=', 4);
expect(3, '<=', 4);

positive: assert that a number is positive

expect(3, 'to be positive');

negative: assert that a number is negative

expect(-1, 'to be negative');

fail: explicitly forces failure.'Custom failure message')'{0} was expected to be {1}', 0, 'zero');

I case you want to rethrow an error, you should always use, as it ensures that the error message will be correct for the different error modes.

var error = new Error('throw me'); Error(error));

When you want to build a completely custom output, you can call with a callback and receive a magicpen instance that the output can be written to. (output) {
    'You have been a very bad boy!'.split(/ /).forEach(function (word, index) {
        if (index > 0) { output.sp(); }
        var style = index % 2 === 0 ? 'cyan' : 'magenta';
Custom error output using

array whose items satisfy: will run an assertion function for each items in an array

expect([0, 1, 2, 3, 4], 'to be an array whose items satisfy', function (item, index) {
    expect(item, 'to be a number');
expect([0, 1, 2, 3, 4], 'to be an array whose items satisfy', 'to be a number');
expect([[1], [2]], 'to be an array whose items satisfy',
       'to be an array whose items satisfy', 'to be a number');
expect([[], []], 'to be a non-empty array whose items satisfy', function (item) {
    expect(item, 'to be an empty array');

Using this assertion result in very detailed error reporting as shown in the below example:

expect([[0, 1, 2], [4, '5', 6], [7, 8, '9']],
       'to be an array whose items satisfy', function (arr) {
    expect(arr, 'to be an array whose items satisfy', function (item) {
        expect(item, 'to be a number');

will output:

failed expectation in [ [ 0, 1, 2 ], [ 4, '5', 6 ], [ 7, 8, '9' ] ]:
  1: failed expectation in [ 4, '5', 6 ]:
       1: expected '5' to be a number
  2: failed expectation in [ 7, 8, '9' ]:
       2: expected '9' to be a number

satisfy: match against a spec

All properties and nested objects mentioned in the right-hand side object are required to be present. Primitive values are compared with to equal semantics:

expect({ hey: { there: true } }, 'to satisfy', { hey: {} });

To disallow additional properties in the subject, use to exhaustively satisfy:

expect({ hey: { there: true } }, 'to exhaustively satisfy', { hey: { there: true } });

Regular expressions and functions in the right-hand side object will be run against the corresponding values in the subject:

expect({ bar: 'quux', baz: true }, 'to satisfy', { bar: /QU*X/i });

Can be combined with to create complex specifications that delegate to existing assertions:

expect({foo: 123, bar: 'bar', baz: 'bogus', qux: 42}, 'to satisfy', {
    foo:'to be a number').and('to be greater than', 10),
    baz:'not to match', /^boh/),
    qux:'to be a string')
                  .and('not to be empty')
               .or('to be a number')
                  .and('to be positive')

map whose keys satisfy: will run an assertion function for each key in a map

expect({ foo: 0, bar: 1, baz: 2, qux: 3 },
       'to be a map whose keys satisfy', function (key) {
    expect(key, 'to match', /^[a-z]{3}$/);
expect({ foo: 0, bar: 1, baz: 2, qux: 3 },
       'to be a map whose keys satisfy',
       'to match', /^[a-z]{3}$/);

Using this assertion result in very detailed error reporting as shown in the below example:

expect({ foo: 0, bar: 1, baz: 2, qux: 3, quux: 4 },
       'to be a map whose keys satisfy', function (key) {
    expect(key, 'to have length', 3);

will output:

failed expectation on keys foo, bar, baz, qux, quux:
  quux: expected 'quux' to have length 3

map whose values satisfy: will run an assertion function for each value in a map

expect({ foo: 0, bar: 1, baz: 2, qux: 3 },
       'to be a map whose values satisfy', function (value) {
    expect(value, 'to be a number');
expect({ foo: 0, bar: 1, baz: 2, qux: 3 },
       'to be a map whose values satisfy',
       'to be a number');

Using this assertion result in very detailed error reporting as shown in the below example:

expect({ foo: [0, 1, 2], bar: [4, '5', 6], baz: [7, 8, '9'] },
       'to be a map whose values satisfy', function (arr) {
    expect(arr, 'to be an array whose items satisfy', function (item) {
        expect(item, 'to be a number');

will output:

failed expectation in
  foo: [ 0, 1, 2 ],
  bar: [ 4, '5', 6 ],
  baz: [ 7, 8, '9' ]
  bar: failed expectation in [ 4, '5', 6 ]:
         1: expected '5' to be a number
  baz: failed expectation in [ 7, 8, '9' ]:
         2: expected '9' to be a number

Extending Unexpected with new assertions


Before extending the expect instance with new assertions it is usually a good idea to clone it, so you don't change the global instance. You do that by calling the clone method on expect. Adding new assertions to the clone will not affect the original instance.

expect.addAssertion([typeName, ...], [pattern, ...], handler)


expect.addAssertion(pattern, handler);
expect.addAssertion([pattern, ...]], handler);
expect.addAssertion(typeName, pattern, handler);
expect.addAssertion(typeName, [pattern, ...], handler);
expect.addAssertion([typeName, ...], pattern, handler);
expect.addAssertion([typeName, ...], [pattern, ...], handler);

New assertions can be added to Unexpected to following way.

expect.addAssertion('array', '[not] to be (sorted|ordered)', function(expect, subject, cmp) {
    expect(subject, '[not] to equal', [].concat(subject).sort(cmp));

The above assertion definition makes the following expects possible:

expect([1,2,3], 'to be sorted');
expect([1,2,3], 'to be ordered');
expect([2,1,3], 'not to be sorted');
expect([2,1,3], 'not to be ordered');
expect([3,2,1], 'to be sorted', function (x, y) { return y - x; });

Let's dissect the different parts of the custom assertion we just introduced.

The first parameter to addAssertion is a string or an array specifying which types the assertion should be defined on. In this case the assertion in only defined for arrays. In case the type is not specified the assertion will be defined for the type any, and would be applicable any type. See the Extending Unexpected with new types section for more information about the type system in Unexpected.

The second parameter to addAssertion is a string or an array stating the patterns this assertion should match. A pattern has the following syntax. A word in square brackets represents a flag that can either be there or not. If the flag is present this.flags[flag] will contain the value true. In this case not is a flag. When a flag it present in a nested expect it will be inserted if the flag is present; otherwise it will be removed. Text that is in parentheses with vertical bars between them are treated as alternative texts that can be used. In this case you can write ordered as an alternative to sorted.

The last parameter to addAssertion is function that will be called when expect is invoked with an expectation matching the type and pattern of the assertion.

So in this case, when expect is called the following way:

expect([3,2,1], 'to be sorted', reverse);

The handler to our assertion will be called with the values the following way, where the not flag in the nested expect will be removed:

expect.addAssertion('[not] to be (sorted|ordered)', function(expect, [3,2,1], reverse){
    expect([3,2,1], '[not] to equal', [].concat([3,2,1]).sort(reverse));

Controlling the output of nested expects

When a call to expect fails inside your assertion the standard error message for the custom assertion will be used. In the case of our sorted assertion the output will be something along the lines:

expected [ 4, 3, 1, 2 ] to be sorted

We can control the output of the nested expects by using the this.errorMode flag.

expect.addAssertion('[not] to be (sorted|ordered)', function(expect, subject, cmp) {
    this.errorMode = 'bubble';
    expect(subject, '[not] to equal', [].concat(subject).sort(cmp));

This will change the error output to:

expected [ 4, 3, 1, 2 ] to equal [ 1, 2, 3, 4 ]

If we change the error mode to nested, we get the following:

expected [ 4, 3, 1, 2 ] to be sorted
  expected [ 4, 3, 1, 2 ] to equal [ 1, 2, 3, 4 ]

The best resource for learning more about custom assertions is to look at how the predefined assertions are build:



Unexpected comes with a type system that is used to explain how different types are compared, diffed, inspected and is also used to limit the scope of assertions.

The following types are provided by out of the box by Unexpected: any, arguments, array, arrayLike, binaryArray, boolean, Buffer, date, Error, function, null, number, object, regexp, string, undefined.


Unexpected can be extended with knowledge about new types by calling the addType method with a type definition. The type definition must implement the required parts of the following interface:

Required members:

  • name: String - the name of the type.
  • identify: boolean function(value) - a function deciding if the type should be used for the given value.

Optional members:

  • base: String - the name of the base type. Defaults to any.
  • equal: boolean function(a, b, equal) - a function capable of comparing two values of this type for equality. If not specified it is inherited from the base type.
  • inspect: function(value, depth, output, inspect) - a function capable of inspecting a value of this type. If not specified it is inherited from the base type.
  • diff: comparison function(actual, expected, output, diff, inspect) - a function producing a comparison between two values of this type. If not specified it is inherited from the base type.


Adding new types to the system is best explained by an example. Let's say we wanted to add first class support for a Person type:

function Person(name, age) { = name;
    this.age = age;

We start out by creating a basic type for handling Person instances. The name of the type should be Person and it should inherit from the build in object type. Furthermore we add an identify method that will recognize Person instances.

    name: 'Person',
    base: 'object',
    identify: function (value) {
        return value instanceof Person;

When you specify a base type, you inherit the optional members you didn't implement. In this case we inherited the methods equal, inspect and diff from the object type.

Imagine that we make a failing expectation on a person instance:

expect(new Person('John Doe', 42), 'to equal', new Person('Jane Doe', 24));

the result is the following output:

Failed expectation of basic type

That is already quite helpful, but the output misses the information that it is Person instances we are comparing. We can fix that by implementing an inspect method on the type.

    name: 'Person',
    base: 'object',
    identify: function (value) {
        return value instanceof Person;
    inspect: function (person, depth, output, inspect) {
       output.text('new Person(')
             .append(inspect(, depth))
             .append(inspect(person.age, depth))

Now we get the following output:

Failed expectation of type with inspect defined

That is a bit better, let me explain how it works. The inspect method is called with the value to be inspected, the depth this type should be inspected with, an output the inspected value should be written to and an inspect function that can be used to recursively inspect members. The output is an instance of magicpen extended with a number of styles.

We write new Person( without styling, then we append the inspected name, write a ,, inspect the age and finish with the closing parenthesis. When inspect is called without a depth parameter it defaults to depth-1. Values inspected with depth zero will be inspected as .... In this case we always want the name so we forward the same depth to the inspect function.

Let's say we wanted persons only to be compared by name and not by age. Then we need to override the equal method:

    name: 'Person',
    base: 'object',
    identify: function (value) {
        return value instanceof Person;
    inspect: function (person, depth, output, inspect) {
       output.text('new Person(')
             .append(inspect(, depth))
             .append(inspect(person.age, depth))
    equal: function (a, b, equal) {
        return a === b || equal(,;

This will produce the same output as above, but that means the diff if wrong. It states that the age should be changed. We can fix that the following way:

    name: 'Person',
    base: 'object',
    identify: function (value) {
        return value instanceof Person;
    inspect: function (person, depth, output, inspect) {
       output.text('new Person(')
             .append(inspect(, depth))
             .append(inspect(person.age, depth))
    equal: function (a, b, equal) {
        return a === b || equal(,;
    diff: function (actual, expected, output, diff, inspect) {
        return this.baseType.diff({name:}, {name:});
Failed expectation of type with diff defined

The above diff method just calls the diff method on the base type with objects that only contain the name. The object diff will take care of all the hard work. We could also have called the diff function we got as an argument, but that will go off detecting the types of the parameters, therefore it is faster to call diff method on the base directly when you know it is the one you need.

You could also do something really custom as seen below:

    name: 'Person',
    base: 'object',
    identify: function (value) {
        return value instanceof Person;
    inspect: function (person, depth, output, inspect) {
       output.text('new Person(')
             .append(inspect(, depth))
             .append(inspect(person.age, depth))
    equal: function (a, b, equal) {
        return a === b || equal(,;
    diff: function (actual, expected, output, diff, inspect) {
        var nameDiff = diff(,;
        output.text('new Person(')
        if (nameDiff && nameDiff.inline) {
        } else {
                  .annotationBlock(function () {
                      this.error('should be ').append(inspect(;
                      if (nameDiff) {
        return {
            inline: false,
            diff: output

That would produce the following output.

Failed expectation of type with an improved diff

This is a rather complicated example and I wont go though the details, but I would like to comment on the inline flag. When we diff objects against each other, the values of the keys will be diffed against each other. That means diffs are inserted into the containing structure. You can control this behavior using the inline flag. If the child diff is inline, it means that it will be appended directly into the parent; otherwise the diff will be inserted in an annotation block. The outputs below shows the contrast between setting the Person diff to inline or not.

Person diff inlining is on Person diff inlining is off

Now that we have implemented a type, we can start adding assertions to it. These assertions will only work on this type or types inheriting from the type.

expect.addAssertion('Person', 'to be above legal age', function (expect, subject) {
    expect(subject.age, 'to be greater than or equal to', 18);
expect(new Person('Jane Doe', 24), 'to be above legal age');

Because Person inherits from object you can use all assertion defined for object or any of it's ancestors. Here is an example:

expect(new Person('Jane Doe', 24), 'to have keys', 'name', 'age');
expect(new Person('Jane Doe', 24), 'to satisfy', {
    name:'to be a string').and('not to be empty'),
    age:'to be a number').and('not to be negative')

The best resource for learning more about custom types is to look at how the predefined types are build:


Plugins - expect.installPlugin(plugin)

Unexpected plugins are objects that adhere to the following interface:

  name: <plugin name>,
  dependencies: <an optional list of dependencies>,
  installInto: <function that will update the given expect instance>

The name of the plugin should be the same at the NPM package name.

A plugin can require a list of other plugins to be installed prior to installation of the plugin. If the dependency list is not fulfilled the installation will fail. The idea is that you manage your plugin versions using NPM. If you install a plugin that is already installed nothing will happen.

The installInto function receives an instance of unexpected and uses uses the addAssertion method to add new custom assertions instance.


See the unexpected-sinon or unexpected-knockout plugins as examples of how to create a plugin.


Prints all registered assertions to the console.


Using Unexpected with a test framework

For example, if you create a test suite with mocha.

Let's say we wanted to test the following program:


function add (a, b) { return a + b; };

Our test file would look like this:

describe('math.js', function () {
  describe('add', function () {
    it('is a function', function () {
      expect(add, 'to be a', 'function');
    it('does addition on numbers', function () {
      expect(add(1, 3), 'to be', 4);

If a certain expectation fails, an exception will be raised which gets captured and shown/processed by the test runner.


Everything you need to know to contribute to unexpected.


MIT, see the LICENSE file for details


Heavily borrows from expect.js by Guillermo Rauch - MIT.


npm i unexpected

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