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2.0.0 • Public • Published

Nightwatch VRT

Nightwatch Visual Regression Testing tools for nightwatch.js


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


Include the following sections in the nightwatch configuration file

Custom commands and assertions

Register nightwatch-vrt's assertion and commands:

    custom_commands_path: [
    custom_assertions_path: [

Nightwatch VRT custom settings

Then, for global settings, add the visual_regression_settings entry to nightwatch's globals globals section

default: {
    "globals": {
        "visual_regression_settings": {
            "generate_screenshot_path": defaultScreenshotPathGenerator,
            "latest_screenshots_path": "vrt/latest",
            "latest_suffix": "",
            "baseline_screenshots_path": "vrt/baseline",
            "baseline_suffix": "",
            "diff_screenshots_path": "vrt/diff",
            "diff_suffix": "",
            "threshold": 0,
            "prompt": false,
            "always_save_diff_screenshot": 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
always_save_diff_screenshot 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)


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.

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
module.exports = {
    'Test Google UI loads correctly': (browser) => {
            .assert.screenshotIdenticalToBaseline('body',  /* Optional */ 'custom-name', {threshold: 0.5}, 'VRT custom-name complete.')

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.




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npm i @bbc/nightwatch-vrt

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