cypress-plugin-snapshots

Plugin for snapshot tests in Cypress.io.

Installation

npm i cypress-plugin-snapshots -S

Usage for text snapshots

describe('data test', () => {
  it('toMatchSnapshot - JSON', () => {
    return cy.request('data.json')
      .its('body')
      .toMatchSnapshot();
  });

  it('toMatchSnapshot - JSON with options', () => {
    return cy.request('data.json')
      .its('body')
      .toMatchSnapshot({
        ignoreExtraFields: true,
      });
  });

  it('toMatchSnapshot - HTML', () => {
    cy.visit('page.html')
      .then(() => {
        cy.get('div').toMatchSnapshot();
      });
  });

  it('toMatchSnapshot - object of type any using cy.wrap()', () => {
    const actualString = "this is an exmample of string data you want to match to a snapshot";
    cy.wrap(actualString).toMatchSnapshot();

    const actualObject = { name: 'Jane Lane' };
    cy.wrap(actualObject).toMatchSnapshot();
  });

  it('toMatchSnapshot - readFile', () => {
    cy.readFile('menu.json').toMatchSnapshot();
  });
});

You can pass the following options to toMatchSnapshot to override default behavior.

{
  "ignoreExtraFields": false,         // Ignore fields that are not in snapshot
  "ignoreExtraArrayItems": false,     // Ignore if there are extra array items in result
  "normalizeJson": true,              // Alphabetically sort keys in JSON
  "replace": {                        // Replace `${key}` in snapshot with `value`.
    "key": "value",
  }
}

replace Use replace with caution. Tests should be deterministic. It's often a better solution to influence your test result instead of your snapshot (by mocking data for example).

Usage for image snapshots

it('toMatchImageSnapshot - element', () => {
  cy.visit('/static/stub.html')
    .then(() => {
      cy.get('[data-test=test]')
        .toMatchImageSnapshot();
    });
});

it('toMatchImageSnapshot - whole page', () => {
  cy.visit('/static/stub.html')
    .then(() => {
      cy.document()
        .toMatchImageSnapshot();
    });
});

You can pass the following options to toMatchImageSnapshot to override default behavior.

{
  "imageConfig": {
    "createDiffImage": true,                // Should a "diff image" be created, can be disabled for performance
    "threshold": 0.01,                      // Amount in pixels or percentage before snapshot image is invalid
    "thresholdType": "percent",             // Can be either "pixel" or "percent"
  },
  "name": "custom image name",            // Naming resulting image file with a custom name rather than concatenating test titles
  "separator": "custom image separator",  // Naming resulting image file with a custom separator rather than using the default ` #`
}

You can also use any option from the cypress.screenshot arguments list.

For example:

cy.get('.element')
  .toMatchImageSnapshot({
    clip: { x: 0, y: 0, width: 100, height: 100 },
  });

Configure Cypress.io

Add this to your cypress.json configuration file:

"ignoreTestFiles": [
  "**/__snapshots__/*",
  "**/__image_snapshots__/*"
]

Plugin

Find your cypress/plugins/index.js file and change it to look like this:

const { initPlugin } = require('cypress-plugin-snapshots/plugin');

module.exports = (on, config) => {
  initPlugin(on, config);
  return config;
};

Command

Find your cypress/support/index.js file and add the following line:

import 'cypress-plugin-snapshots/commands';

Make changes to default configuration

You can customize the configuration in the cypress.json file in the root of your Cypress project.

Add the configuration below to your cypress.json file to make changes to the default values.

"env": {
  "cypress-plugin-snapshots": {
    "autoCleanUp": false,            // Automatically remove snapshots that are not used by test
    "autopassNewSnapshots": true,    // Automatically save & pass new/non-existing snapshots
    "autofailNewSnapshots": false,   // Automatically fail if no existing snapshot found (useful for production builds to prevent tests of passing just because they're missing snapshots)  
    "diffLines": 3,                  // How many lines to include in the diff modal
    "excludeFields": [],             // Array of fieldnames that should be excluded from snapshot
    "ignoreExtraArrayItems": false,  // Ignore if there are extra array items in result
    "ignoreExtraFields": false,      // Ignore extra fields that are not in `snapshot`
    "normalizeJson": true,           // Alphabetically sort keys in JSON
    "prettier": true,                // Enable `prettier` for formatting HTML before comparison
    "imageConfig": {
      "createDiffImage": true,       // Should a "diff image" be created, can be disabled for performance
      "resizeDevicePixelRatio": true,// Resize image to base resolution when Cypress is running on high DPI screen, `cypress run` always runs on base resolution
      "threshold": 0.01,             // Amount in pixels or percentage before snapshot image is invalid
      "thresholdType": "percent"     // Can be either "pixels" or "percent"
    },
    "screenshotConfig": {            // See https://docs.cypress.io/api/commands/screenshot.html#Arguments
      "blackout": [],
      "capture": "fullPage",
      "clip": null,
      "disableTimersAndAnimations": true,
      "log": false,
      "scale": false,
      "timeout": 30000
    },
    "serverEnabled": true,           // Enable "update snapshot" server and button in diff modal
    "serverHost": "localhost",       // Hostname for "update snapshot server"
    "serverPort": 2121,              // Port number for  "update snapshot server"
    "updateSnapshots": false,        // Automatically update snapshots, useful if you have lots of changes
    "backgroundBlend": "difference"  // background-blend-mode for diff image, useful to switch to "overlay"
  }
}

Caveats :warning:

There is currently an issue when running "All Tests" in Cypress with this plugin. You can follow the progress on the issue here and here. When running "All Tests" any tests that utilize cypress-plugin-snapshots will throw an error.

Roadmap

Below is a list of functionality that is under consideration for implementing in a next version.

• Fix handling of "update snapshot" button that contains a replacable field
• Disable "update snapshots" server in headless mode
• Add more unit tests
• Add JSDoc documentation
• Improve TypeScript bindings

Contributing

In lieu of a formal styleguide, take care to maintain the existing coding style.

License

This plugin is released under the MIT license.