Awesome

Power Assert in JavaScript. Provides descriptive assertion messages through standard assert interface. No API is the best API.

DESCRIPTION

What is power-assert?

is an implementation of "Power Assert" concept in JavaScript.
provides descriptive assertion messages through standard assert interface.
No API is the best API. With power-assert, you don't need to learn many assertion library APIs (in most cases, all you need to remember is just an assert(any_expression) function)
Stop memorizing tons of assertion APIs. Just create expressions that return a truthy value or not and power-assert will show it to you right on the screen as part of your failure message without you having to type in a message at all.
the core value of power-assert is absolute simplicity and stability. Especially, power-assert sticks to the simplest form of testing, assert(any_expression).
see slides: "power-assert, mechanism and philosophy" -- talk at NodeFest 2014.
[NEW] Now you don't need require('power-assert') any more. Keep using require('assert'), and power-assert enhances them transparently. See slides: From Library to Tool - power-assert as a General Purpose Assertion Enhancement Tool
to gain power-assert output, you need to transform your test code to produce power-assert output.
power-assert - power = assert. Without code transpilation, power-assert works just as normal assert does.
fully compatible with assert. So you can stop using power-assert and back to assert easily.
has online demo site.
works both on server side and browser side.
available via npm and bower.
supports sourcemaps so you can debug as usual.
provides babel plugin and babel preset.
provides browserify transform.
provides webpack loader.
provides grunt task and gulp plugin.
provides command.
provides custom module loader and its convenient config module.
provides Karma Adapter and Karma Preprocessor.
supports ES6+ through babel plugin.
supports CoffeeScript.
supports TypeScript.
has TypeScript type definition
has code migration tool that transforms existing code from chai, should.js and expect.js to assert.
AVA, the futuristic test runner, now comes with power-assert builtin
has Lab transformer to enable power-assert on Lab
has module loader to get ts-node working together with power-assert
Wallaby.js supports power-assert via Babel compiler/preprocessor
pull-requests, issue reports and patches are always welcomed.

power-assert provides descriptive assertion messages for your tests, like this.

  1) Array #indexOf() should return index when the value is present:
     AssertionError: # path/to/test/mocha_node.js:10

  assert(ary.indexOf(zero) === two)
         |   |       |     |   |
         |   |       |     |   2
         |   -1      0     false
         [1,2,3]

  [number] two
  => 2
  [number] ary.indexOf(zero)
  => -1

API

power-assert enhances these assert functions by espower. Produces descriptive message when assertion is failed.

power-assert is fully compatible with assert. So functions below are also available though they are not enhanced (does not produce descriptive message).

Since version 1.5.0, power-assert supports "strict mode" as well.

power-assert provides an API for customization.

assert.customize(options)

No API is the best API

Though power-assert is fully compatible with standard assert interface, all you need to remember is just an assert(any_expression) function in most cases.

The core value of power-assert is absolute simplicity and stability. Especially, power-assert sticks to the simplest form of testing, assert(any_expression).

    assert(types[index].name === bob.name)
           |    ||      |    |   |   |
           |    ||      |    |   |   "bob"
           |    ||      |    |   Person{name:"bob",age:5}
           |    ||      |    false
           |    |11     "alice"
           |    Person{name:"alice",age:3}
           ["string",98.6,true,false,null,undefined,#Array#,#Object#,NaN,Infinity,/^not/,#Person#]
  
    --- [string] bob.name
    +++ [string] types[index].name
    @@ -1,3 +1,5 @@
    -bob
    +alice

FAQ

INSTALL

npm install --save-dev power-assert <one of instrumentors>

bower install --save-dev power-assert
npm install --save-dev <one of instrumentors>

see list of instrumentors

CHANGELOG

See CHANGELOG

EXAMPLE

See HOW TO USE section for more details.

Note: There is an online demo site available.

Target test code (using Mocha in this example)

'use strict';

const assert = require('assert');

describe('Array', function(){
    let ary;
    beforeEach(() => {
        ary = [1,2,3];
    });
    describe('#indexOf()', () => {
        it('should return index when the value is present', () => {
            const zero = 0, two = 2;
            assert(ary.indexOf(zero) === two);
        });
        it('should return -1 when the value is not present', () => {
            const minusOne = -1, two = 2;
            assert.ok(ary.indexOf(two) === minusOne, 'THIS IS AN ASSERTION MESSAGE');
        });
    });
});

describe('various types', () => {
    let types;
    class Person {
        constructor(name, age) {
            this.name = name;
            this.age = age;
        }
    }
    beforeEach(() => {
        types = [
            'string', 98.6, true, false, null, undefined,
            ['nested', 'array'],
            {object: true},
            NaN, Infinity,
            /^not/,
            new Person('alice', 3)
        ];
    });
    it('demo', () => {
        const index = types.length -1,
            bob = new Person('bob', 5);
        assert(types[index].name === bob.name);
    });
});

Be sure to transform test code

To use power-assert, you need to transform your test code for power-assert output.

Code transform is done by instrumentors below:

If you are using Node.js only, the easiest way is to use intelli-espower-loader. Steps are as follows.

Setup

npm install --save-dev mocha power-assert intelli-espower-loader

Run

Put tests into test directory then run. You will see the power-assert output appears.

  $ $(npm bin)/mocha --require intelli-espower-loader path/to/test/mocha_node.js


  Array
    #indexOf()
      1) should return index when the value is present
      2) should return -1 when the value is not present

  various types
    3) demo


  0 passing (43ms)
  3 failing

  1) Array #indexOf() should return index when the value is present:

      AssertionError:   # test/example2.js:13

  assert(ary.indexOf(zero) === two)
         |   |       |     |   |
         |   |       |     |   2
         |   -1      0     false
         [1,2,3]

  [number] two
  => 2
  [number] ary.indexOf(zero)
  => -1

      + expected - actual

      -false
      +true

      at Context.it (test/example2.js:13:13)

  2) Array #indexOf() should return -1 when the value is not present:

      AssertionError: THIS IS AN ASSERTION MESSAGE   # test/example2.js:17

  assert.ok(ary.indexOf(two) === minusOne, 'THIS IS AN ASSERTION MESSAGE')
            |   |       |    |   |
            |   |       |    |   -1
            |   1       2    false
            [1,2,3]

  [number] minusOne
  => -1
  [number] ary.indexOf(two)
  => 1

      + expected - actual

      -false
      +true

      at Context.it (test/example2.js:17:20)

  3) various types demo:

      AssertionError:   # test/example2.js:43

  assert(types[index].name === bob.name)
         |    ||      |    |   |   |
         |    ||      |    |   |   "bob"
         |    ||      |    |   Person{name:"bob",age:5}
         |    ||      |    false
         |    |11     "alice"
         |    Person{name:"alice",age:3}
         ["string",98.6,true,false,null,undefined,#Array#,#Object#,NaN,Infinity,/^not/,#Person#]

  --- [string] bob.name
  +++ [string] types[index].name
  @@ -1,3 +1,5 @@
  -bob
  +alice


      + expected - actual

      -false
      +true

      at Context.it (test/example2.js:43:9)

SEED PROJECTS

Some seed projects are available to help you start with power-assert.

module	env	tech stack
power-assert-node-seed	Node.js	power-assert + intelli-espower-loader
power-assert-testem-seed	Browsers(by testem)	power-assert + gulp-espower + testem.
power-assert-karma-seed	Browsers(by Karma)	power-assert + espowerify + browserify + Karma.

HOW TO USE

There are some ways to use power-assert. (If you want to see running examples, see SEED PROJECTS)

power-assert + Babel + babel-preset-power-assert: The only way to enable power-assert if you are using Babel6+.
power-assert + espower-loader or intelli-espower-loader : Simple and recommended (but only works under Node).
power-assert + espower-coffee or espower-typescript: Use power-assert with AltJS. Recommended but only works under Node.
power-assert + browserify + espowerify: if you are using browserify but not with Babel.
power-assert + webpack + webpack-espower-loader: if you are using webpack but not with Babel.
power-assert + espower-cli or grunt-espower or gulp-espower : Generate instrumented code so works anywhere.

using `babel-preset-power-assert` or `babel-plugin-espower`

If you are writing your code with Babel, you can instrument Power Assert feature with Babel and babel-preset-power-assert (or babel-plugin-espower).

see babel-plugin-espower README and babel-preset-power-assert README

using `espower-loader` or `intelli-espower-loader`

If you are writing Node.js app/module, you can instrument Power Assert feature without code generation by using espower-loader.

see espower-loader README.

FYI: You may be interested in intelli-espower-loader to go one step further. With intelli-espower-loader, you don't need to create loader file (like enable-power-assert.js). Just define test directory in package.json wow!

using `espower-typescript`

If you are writing Node.js app/module in TypeScript, you can instrument Power Assert feature without code generation by using espower-typescript.

see espower-typescript README.

using `espower-coffee`

If you are writing Node.js app/module in CoffeeScript, you can instrument Power Assert feature without code generation by using espower-coffee.

see espower-coffee README.

using `espowerify`

If you are using browserify but not with Babel, you can instrument Power Assert feature via espowerify.

see espowerify README.

using `webpack-espower-loader`

If you are using webpack but not with Babel, you can instrument Power Assert feature via webpack-espower-loader.

see webpack-espower-loader README.

using `espower-cli`

If you don't want to use grunt, gulp, browserify, and so on, you can use power-assert via bower, with generated code by espower-cli

see espower-cli README.

using `gulp-espower`

On the browser side and you are not using browserify but bower and gulp, you can use power-assert via bower, with generated code by gulp-espower

see gulp-espower README.

using `grunt-espower`

On the browser side and you are not using browserify but bower and Grunt, you can use power-assert via bower, with generated code by grunt-espower

see grunt-espower README.

CUSTOMIZATION API

power-assert provides an API for customization.

var assert = assert.customize(options)

Through this API, you can customize power-assert by changing some options.

var assert = require('power-assert').customize({
    output: {
        maxDepth: 2
    }
});

options

options has two top-level keys. assertion and output.

options.assertion

customization options for empower module. See empower API documentation for details. Note that some default values are different from empower's (modifyMessageOnRethrow: true and saveContextOnRethrow: true).

options.output

customization options for power-assert-formatter module. See power-assert-formatter API documentation for details.

default values

customizable properties and their default values are as follows.

var assert = require('power-assert').customize({
    assertion: {
        destructive: false,
        modifyMessageOnRethrow: true,
        saveContextOnRethrow: true,
        patterns: [
            'assert(value, [message])',
            'assert.ok(value, [message])',
            'assert.equal(actual, expected, [message])',
            'assert.notEqual(actual, expected, [message])',
            'assert.strictEqual(actual, expected, [message])',
            'assert.notStrictEqual(actual, expected, [message])',
            'assert.deepEqual(actual, expected, [message])',
            'assert.notDeepEqual(actual, expected, [message])',
            'assert.deepStrictEqual(actual, expected, [message])',
            'assert.notDeepStrictEqual(actual, expected, [message])'
        ]
    },
    output: {
        lineDiffThreshold: 5,
        maxDepth: 1,
        anonymous: 'Object',
        circular: '#@Circular#',
        lineSeparator: '\n',
        ambiguousEastAsianCharWidth: 2,
        widthOf: (Function to calculate width of string. Please see power-assert-formatter's documentation)
        stringify: (Function to stringify any target value. Please see power-assert-formatter's documentation)
        diff: (Function to create diff string between two strings. Please see power-assert-formatter's documentation)
        writerClass: (Constructor Function for output writer class. Please see power-assert-formatter's documentation)
        renderers: [
            './built-in/file',
            './built-in/assertion',
            './built-in/diagram',
            './built-in/binary-expression'
        ]
    }
});

INTERNAL DESIGN

power-assert family provides 1 main module, 4 core modules and many more instrumentors.

Main (facade) module is,

module	description
power-assert	Standard `assert` function on top of `empower` and `power-assert-formatter`

core modules are,

module	description
empower	Power Assert feature enhancer for assert function/object.
power-assert-formatter	Power Assert output formatter.
espower	Power Assert feature instrumentor core based on the ECMAScript AST defined in The ESTree Spec (formerly known as Mozilla SpiderMonkey Parser API).
espower-source	Power Assert instrumentor from source to source, with source-map. (Thin wrapper of `espower`).

and instrumentors are,

module	description
espower-loader	Node module loader to apply `espower` on the fly.
intelli-espower-loader	configure `espower-loader` with ease.
babel-preset-power-assert	Babel preset to instrument power-assert feature into target files.
babel-plugin-espower	Babel plugin to instrument power-assert feature into target files.
espowerify	Browserify transform to apply `espower` to target files.
webpack-espower-loader	Power Assert instrumentor module for webpack.
espower-cli	Command line tool for power-assert.
grunt-espower	Grunt task to apply `espower` to target files.
gulp-espower	Gulp plugin to apply `espower` to target files.
karma-espower-preprocessor	karma-preprocessor for power-assert.
espower-coffee	power-assert instrumentor for CoffeeScript.
espower-typescript	power-assert instrumentor for TypeScript.
espower-traceur	power-assert instrumentor for ES6 using Traceur Compiler.
espower-babel	[DEPRECATED] power-assert instrumentor for ES6 using Babel.

power-assert provides standard assert compatible function with Power Assert feature. (Best fit with Mocha. If you use assert-like objects provided by various testing frameworks such as QUnit or nodeunit. Please use empower and power-assert-formatter modules directly).

Internally, power-assert uses empower module to enhance power assert feature into the standard assert module, to run with the power assert feature added code by espower module, and prettify output using power-assert-formatter.

See power-assert-demo project for power-assert Demo running with mocha.

SUPPORTED FRAMEWORKS

Mocha
AVA

FRAMEWORKS KNOWN TO WORK

Jest with Babel
Karma with Mocha
testem with Mocha
QUnit
nodeunit
buster-assertions
Lab
Nightmare
Protractor
eater

OUR SUPPORT POLICY

For the Transpiler side, we support Node under maintenance. In other words, we stop supporting old Node version when their maintenance ends.

For the Runtime side, we support Node under maintenance and "modern enough" browsers such as Chrome, Firefox, Safari, Edge etc.

Any other environments are not supported officially (means that we do not test against them on CI service). power-assert is known to work with old browsers, and trying to keep them working though.

AUTHOR

Takuto Wada

CONTRIBUTORS

LICENSE

Licensed under the MIT license.

MORE OUTPUT EXAMPLES

Target test code (using QUnit in this example)

var q = require('qunitjs');

(function () {
    var empower = require('empower'),
        formatter = require('power-assert-formatter'),
        qunitTap = require("qunit-tap");
    empower(q.assert, formatter(), {destructive: true});
    qunitTap(q, require('util').puts, {showSourceOnFailure: false});
    q.config.autorun = false;
})();

q.test('spike', function (assert) {
    assert.ok(true);

    var hoge = 'foo';
    var fuga = 'bar';
    assert.ok(hoge === fuga, 'comment');

    var piyo = 3;
    assert.ok(fuga === piyo);

    var longString = 'very very loooooooooooooooooooooooooooooooooooooooooooooooooooong message';
    var anotherLongString = 'yet another loooooooooooooooooooooooooooooooooooooooooooooooooooong message';
    assert.ok(longString === anotherLongString);

    assert.ok(4 === piyo);

    assert.ok(4 !== 4);

    var falsyStr = '';
    assert.ok(falsyStr);

    var falsyNum = 0;
    assert.ok(falsyNum);

    var ary1 = ['foo', 'bar'];
    var ary2 = ['aaa', 'bbb', 'ccc'];
    assert.ok(ary1.length === ary2.length);
    assert.deepEqual(ary1, ary2);

    var actual = 16;
    assert.ok(5 < actual && actual < 13);

    actual = 4;
    assert.ok(5 < actual && actual < 13);

    actual = 10;
    assert.ok(actual < 5 || 13 < actual);


    var propName = 'bar',
        foo = {
            bar: {
                baz: false
            }
        };

    assert.ok(foo.bar.baz);
    assert.ok(foo['bar'].baz);
    assert.ok(foo[propName]['baz']);


    var truth = true;
    assert.ok(!truth);


    var func = function () { return false; };
    assert.ok(func());


    var obj = {
        age: function () {
            return 0;
        }
    };
    assert.ok(obj.age());


    var isFalsy = function (arg) {
        return !(arg);
    };
    var positiveInt = 50;
    assert.ok(isFalsy(positiveInt));


    var sum = function () {
        var result = 0;
        for (var i = 0; i < arguments.length; i += 1) {
            result += arguments[i];
        }
        return result;
    };
    var one = 1, two = 2, three = 3, seven = 7, ten = 10;
    assert.ok(sum(one, two, three) === seven);
    assert.ok(sum(sum(one, two), three) === sum(sum(two, three), seven));
    assert.ok((three * (seven * ten)) === three);


    var math = {
        calc: {
            sum: function () {
                var result = 0;
                for (var i = 0; i < arguments.length; i += 1) {
                    result += arguments[i];
                }
                return result;
            }
        }
    };
    assert.ok(math.calc.sum(one, two, three) === seven);
});

q.load();

`espower` code above then running under Node.js

# module: undefined
# test: spike
ok 1 - okay
not ok 2 - comment # path/to/examples/qunit_node.js:17
#
# assert.ok(hoge === fuga, 'comment')
#           |    |   |
#           |    |   "bar"
#           |    false
#           "foo"
#
# --- [string] fuga
# +++ [string] hoge
# @@ -1,3 +1,3 @@
# -bar
# +foo
#
# , test: spike
not ok 3 - # path/to/examples/qunit_node.js:20
#
# assert.ok(fuga === piyo)
#           |    |   |
#           |    |   3
#           |    false
#           "bar"
#
# [number] piyo
# => 3
# [string] fuga
# => "bar"

# , test: spike
not ok 4 - # path/to/examples/qunit_node.js:24
#
# assert.ok(longString === anotherLongString)
#           |          |   |
#           |          |   "yet another loooooooooooooooooooooooooooooooooooooooooooooooooooong message"
#           |          false
#           "very very loooooooooooooooooooooooooooooooooooooooooooooooooooong message"
#
# --- [string] anotherLongString
# +++ [string] longString
# @@ -1,15 +1,13 @@
# -yet anoth
# +very v
#  er
# +y
#   loo
#
# , test: spike
not ok 5 - # path/to/examples/qunit_node.js:26
#
# assert.ok(4 === piyo)
#             |   |
#             |   3
#             false
#
# [number] piyo
# => 3
# [number] 4
# => 4
# , test: spike
not ok 6 - # path/to/examples/qunit_node.js:28
#
# assert.ok(4 !== 4)
#             |
#             false
# , test: spike
not ok 7 - # path/to/examples/qunit_node.js:31
#
# assert.ok(falsyStr)
#           |
#           ""
# , test: spike
not ok 8 - # path/to/examples/qunit_node.js:34
#
# assert.ok(falsyNum)
#           |
#           0
# , test: spike
not ok 9 - # path/to/examples/qunit_node.js:38
#
# assert.ok(ary1.length === ary2.length)
#           |    |      |   |    |
#           |    |      |   |    3
#           |    |      |   ["aaa","bbb","ccc"]
#           |    2      false
#           ["foo","bar"]
#
# [number] ary2.length
# => 3
# [number] ary1.length
# => 2
# , test: spike
not ok 10 - # path/to/examples/qunit_node.js:39
#
# assert.deepEqual(ary1, ary2)
#                  |     |
#                  |     ["aaa","bbb","ccc"]
#                  ["foo","bar"]
# , expected: [
#   "aaa",
#   "bbb",
#   "ccc"
# ], got: [
#   "foo",
#   "bar"
# ], test: spike
not ok 11 - # path/to/examples/qunit_node.js:42
#
# assert.ok(5 < actual && actual < 13)
#             | |      |  |      |
#             | |      |  16     false
#             | 16     false
#             true
# , test: spike
not ok 12 - # path/to/examples/qunit_node.js:45
#
# assert.ok(5 < actual && actual < 13)
#             | |      |
#             | 4      false
#             false
# , test: spike
not ok 13 - # path/to/examples/qunit_node.js:48
#
# assert.ok(actual < 5 || 13 < actual)
#           |      |   |     | |
#           |      |   |     | 10
#           |      |   false false
#           10     false
# , test: spike
not ok 14 - # path/to/examples/qunit_node.js:58
#
# assert.ok(foo.bar.baz)
#           |   |   |
#           |   |   false
#           |   Object{baz:false}
#           Object{bar:#Object#}
# , test: spike
not ok 15 - # path/to/examples/qunit_node.js:59
#
# assert.ok(foo['bar'].baz)
#           |  |       |
#           |  |       false
#           |  Object{baz:false}
#           Object{bar:#Object#}
# , test: spike
not ok 16 - # path/to/examples/qunit_node.js:60
#
# assert.ok(foo[propName]['baz'])
#           |  ||        |
#           |  |"bar"    false
#           |  Object{baz:false}
#           Object{bar:#Object#}
# , test: spike
not ok 17 - # path/to/examples/qunit_node.js:64
#
# assert.ok(!truth)
#           ||
#           |true
#           false
# , test: spike
not ok 18 - # path/to/examples/qunit_node.js:68
#
# assert.ok(func())
#           |
#           false
# , test: spike
not ok 19 - # path/to/examples/qunit_node.js:76
#
# assert.ok(obj.age())
#           |   |
#           |   0
#           Object{age:#function#}
# , test: spike
not ok 20 - # path/to/examples/qunit_node.js:83
#
# assert.ok(isFalsy(positiveInt))
#           |       |
#           false   50
# , test: spike
not ok 21 - # path/to/examples/qunit_node.js:94
#
# assert.ok(sum(one, two, three) === seven)
#           |   |    |    |      |   |
#           |   |    |    |      |   7
#           6   1    2    3      false
#
# [number] seven
# => 7
# [number] sum(one, two, three)
# => 6
# , test: spike
not ok 22 - # path/to/examples/qunit_node.js:95
#
# assert.ok(sum(sum(one, two), three) === sum(sum(two, three), seven))
#           |   |   |    |     |      |   |   |   |    |       |
#           |   |   |    |     |      |   12  5   2    3       7
#           6   3   1    2     3      false
#
# [number] sum(sum(two, three), seven)
# => 12
# [number] sum(sum(one, two), three)
# => 6
# , test: spike
not ok 23 - # path/to/examples/qunit_node.js:96
#
# assert.ok(three * (seven * ten) === three)
#           |     |  |     | |    |   |
#           |     |  |     | |    |   3
#           |     |  |     | 10   false
#           |     |  7     70
#           3     210
#
# [number] three
# => 3
# [number] three * (seven * ten)
# => 210
# , test: spike
not ok 24 - # path/to/examples/qunit_node.js:110
#
# assert.ok(math.calc.sum(one, two, three) === seven)
#           |    |    |   |    |    |      |   |
#           |    |    |   |    |    |      |   7
#           |    |    6   1    2    3      false
#           |    Object{sum:#function#}
#           Object{calc:#Object#}
#
# [number] seven
# => 7
# [number] math.calc.sum(one, two, three)
# => 6
# , test: spike
1..24

Have fun!