@nightwatch/vrt NPM

@nightwatch/vrt

Official Nightwatch plugin that adds visual regression testing support.

Installation

Step 1 – install from NPM

npm i @nightwatch/vrt --save-dev

Step 2 – add the plugin to the list

Update your Nightwatch configuration and add the plugin to the list:

module.exports = {
  plugins: ['@nightwatch/vrt']
    
  // other nightwath settings...
}

Usage

Nightwatch VRT extends Nightwatch.js with an assertion that captures a screenshot of a DOM element identified by a selector and compares it against a baseline. If the baseline screenshot does not exist, it will be created the first time you run the test and the assertion will pass.

VRT custom settings

The @nightwatch/vrt plugin comes by default with sensible configuration, but in some scenarios you may need to change some of the config options.

You can change the settings by adding @nightwatch/vrt entry to Nightwatch config

//nightwatch.conf.js

module.exports = {
    
    //... other config

   '@nightwatch/vrt': {
    latest_screenshots_path: 'vrt/latest',
    latest_suffix: '',
    baseline_screenshots_path: 'vrt/baseline',
    baseline_suffix: '',
    diff_screenshots_path: 'vrt/diff',
    diff_suffix: '',
    threshold: 0.01,
    prompt: false,
    updateScreenshots: false
  },
}

Property	Description	Defaults
generate_screenshot_path	Passed function that will generate a screenshot path	none
latest_screenshots_path	Path to the most recently captured screenshots	"vrt/latest"
latest_suffix	A string appended to the end of the latest captured screenshot*	""
baseline_screenshots_path	Path to the baseline expected screenshots	"vrt/baseline"
baseline_suffix	A string appended to the end of the baseline screenshot*	""
diff_screenshots_path	Path to the diff image of the two screenshots	"vrt/diff"
diff_suffix	A string appended to the end of the diff image*	""
threshold	Matching threshold, ranges from `0` to `1`. Smaller values make the comparison more sensitive.	0.0
prompt	If true, the user will be prompted to override baseline screenshot when the recently captured screenshot differs	false
updateScreenshots	If true, recently captured screenshots will always override the baseline	false

* Only necessary if screenshots are set to reside in the same directory

Nightwatch VRT screenshot path generator

The screenshot path generator option accepts a function that generates a dynamic path based on the test properties, and returns that string.

Parameter	Description
nightwatchClient	The nightwatch client test instance
basePath	The base path for the screenshot set in `visual_regression_settings` (e.g. *_screenshots_path)
fileName	The file name; either the selector used or the custom name given for the test
*returns*	A string which contains the full path - minus the file extension

For example:

function generateScreenshotFilePath(nightwatchClient, basePath, fileName) {
    const moduleName = nightwatchClient.currentTest.module,
        testName = nightwatchClient.currentTest.name

    return path.join(process.cwd(), basePath, moduleName, testName, fileName)
}

API Commands

In order to use nightwatch-vrt, you only need to invoke the screenshotIdenticalToBaseline assertion and pass a css selector for the DOM element to compare. You may also pass a custom filename, visual_regression_settings overrides, and a custom log message.

- assert.screenshotIdenticalToBaseline

Parameter	Description
selector	Identifies the element that will be captured in the screenshot.
fileName	Optional file name for this screenshot; defaults to the selector
settings	Optional settings to override the defaults and `visual_regression_settings`
message	Optional message for `nightwatch` to log upon completion

describe('VRT demo test', function() {
    it('Test Google UI loads correctly', function(browser) {
        browser
            .url('https://www.google.co.uk')
            .assert.screenshotIdenticalToBaseline('body',  /* Optional */ 'custom-name', {threshold: 0.0}, 'VRT custom-name complete.')
            .end()
    })
})

The first time a test is run, a baseline screenshot will be created and stored on disk. You should always register the baseline screenshot in the code repository. Further executions of this test will compare against this baseline.