expect.js

What is expect.js?

Expect.js is a minimalistic assertion library for Node.js and the browser. It provides a simple and expressive syntax for writing tests, allowing developers to assert conditions in their code.

What are expect.js's main functionalities?

Assertions

Expect.js allows you to assert that a value is of a certain type. In this example, it checks if the value 5 is a number.

expect(5).to.be.a('number');

Equality

You can assert that two values are equal using expect.js. This example checks if the number 5 is equal to 5.

expect(5).to.equal(5);

Truthiness

Expect.js can be used to assert the truthiness of a value. Here, it checks if the value true is truthy.

expect(true).to.be.ok();

Existence

This feature allows you to assert whether a value exists or not. In this example, it checks that the value undefined does not exist.

expect(undefined).to.not.exist();

Length

Expect.js can assert the length of arrays or strings. This example checks if the array [1, 2, 3] has a length of 3.

expect([1, 2, 3]).to.have.length(3);

Other packages similar to expect.js

chai

Chai is a popular assertion library for Node.js and the browser that offers a more extensive API than expect.js. It supports BDD and TDD styles and provides more flexibility with plugins and additional features.

jest

Jest is a comprehensive testing framework that includes a built-in assertion library. It offers more features than expect.js, such as mocking and snapshot testing, making it suitable for larger projects.

should

Should.js is another assertion library that provides a similar expressive syntax to expect.js. It offers more extensive chaining and additional features, making it a more powerful alternative for complex assertions.

README

Expect

Minimalistic BDD assertion toolkit based on should.js

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

Features

• Cross-browser: works on IE6+, Firefox, Safari, Chrome, Opera.
• Compatible with all test frameworks.
• Node.JS ready (require('expect.js')).
• Standalone. Single global with no prototype extensions or shims.

How to use

Node

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

$ npm install expect.js

Then:

var expect = require('expect.js');

Browser

Expose the expect.js found at the top level of this repository.

<script src="expect.js"></script>

API

ok: asserts that the value is truthy or not

expect(1).to.be.ok();
expect(true).to.be.ok();
expect({}).to.be.ok();
expect(0).to.not.be.ok();

be / equal: asserts === equality

expect(1).to.be(1)
expect(NaN).not.to.equal(NaN);
expect(1).not.to.be(true)
expect('1').to.not.be(1);

eql: asserts loose equality that works with objects

expect({ a: 'b' }).to.eql({ a: 'b' });
expect(1).to.eql('1');

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

// typeof with optional `array`
expect(5).to.be.a('number');
expect([]).to.be.an('array');  // works
expect([]).to.be.an('object'); // works too, since it uses `typeof`

// constructors
expect(5).to.be.a(Number);
expect([]).to.be.an(Array);
expect(tobi).to.be.a(Ferret);
expect(person).to.be.a(Mammal);

match: asserts String regular expression match

expect(program.version).to.match(/[0-9]+\.[0-9]+\.[0-9]+/);

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);

empty: asserts that an array is empty or not

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

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

expect(window).to.have.property('expect')
expect(window).to.have.property('expect', expect)
expect({a: 'b'}).to.have.property('a');

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

expect({ a: 'b' }).to.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' }).to.not.only.have.key('a');

throwException/throwError: asserts that the Function throws or not when called

expect(fn).to.throwError(); // synonym of throwException
expect(fn).to.throwException(function (e) { // get the exception object
  expect(e).to.be.a(SyntaxError);
});
expect(fn).to.throwException(/matches the exception message/);
expect(fn2).to.not.throwException();

withArgs: creates anonymous function to call fn with arguments

expect(fn).withArgs(invalid, arg).to.throwException();
expect(fn).withArgs(valid, arg).to.not.throwException();

within: asserts a number within a range

expect(1).to.be.within(0, Infinity);

greaterThan/above: asserts >

expect(3).to.be.above(0);
expect(5).to.be.greaterThan(3);

lessThan/below: asserts <

expect(0).to.be.below(3);
expect(1).to.be.lessThan(3);

fail: explicitly forces failure.

expect().fail()
expect().fail("Custom failure message")

Using with a test framework

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

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

math.js

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.equal(4);
  });
});

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

Differences with should.js

• No need for static should methods like should.strictEqual. For example, expect(obj).to.be(undefined) works well.
• Some API simplifications / changes.
• API changes related to browser compatibility.

Running tests

Clone the repository and install the developer dependencies:

git clone git://github.com/LearnBoost/expect.js.git expect
cd expect && npm install

Node

make test

Browser

make test-browser

and point your browser(s) to http://localhost:3000/test/

Credits

(The MIT License)

Copyright (c) 2011 Guillermo Rauch <[e-mail]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

3rd-party

Heavily borrows from should.js by TJ Holowaychuck - MIT.