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4.0.0 • Public • Published


Minimalistic 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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  • 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 IE6+, Firefox, Safari, Chrome, Opera.

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');

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({}, 'to be NaN');
expect(2, 'not to be NaN');
expect(null, 'not to be NaN');
expect(undefined, 'to be NaN');
expect("String", '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');
expect(null, 'not 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 is empty or not

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

finite/infinite: asserts a finite or infinite number

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

expect.fail('Custom failure message')
expect.fail('{0} was expected to be {1}', 0, 'zero');

array whose items satify: 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 show 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'

map whose keys satify: 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 show 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 satify: 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 show 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(...assertionString, handler)

Warning: if you were an early adopter and used addAssertion before it was made public, the API has change slightly to allow more advanced assertions.

New assertions can be added to Unexpected to following way.

expect.addAssertion('[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 describing the different expectation strings this custom assertion should match. 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 is the custom assertion has that flag; 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 one of the expectation strings generated from the custom assertion. When the expect function is called the following way:

expect(testSubject, expectationString, ...arguments);

The expectation string is used to identify a handler for the expectation. The handler is then called with an instance of expect that can be used inside the custom assertion, the test subject and the rest of the arguments.

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

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

The handler to our custom assertion will be called with the values this 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 an expect fails inside your custom 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 plugins are just functions that uses the addAssertion method to add new custom assertions to the expect instance.


See the unexpected-sinon plugin as an example on how to create a plugin.

Print 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('test suite', function () {
  it('should expose a function', function () {
    expect(add, 'to be a', 'function');
  it('should do math', 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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