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Assertions

Usage

Configuring assertions for Lighthouse CI is most easily done through a lighthouserc.json file. While the file supports configuration for many different Lighthouse functions, the settings for your assertions will be found in the ci.assert property of your JSON file. For example,

{
  "ci": {
    "assert": {
      "assertions": {
        "uses-responsive-images": "error"
      }
    }
  }
}

The ci.assert wrapper will be left out of future code samples in this file to simplify the example.

Audits

The result of any audit in Lighthouse can be asserted. Assertions are keyed by the Lighthouse audit ID and follow an eslint-style format of level | [level, options]. For a reference of the audit IDs in each category, you can take a look at the default Lighthouse config. When no options are set, the default options of {"aggregationMethod": "optimistic", "minScore": 1} are used.

{
  "assertions": {
    "first-contentful-paint": "off",
    "works-offline": ["warn", {"minScore": 1}],
    "uses-responsive-images": ["error", {"maxLength": 0}]
  }
}

Categories

The score of any category in Lighthouse can also be asserted. Assertions are keyed by categories:<categoryId> and follow the same eslint-style format as audit assertions. Note that this just affects the category score and will not affect any assertions on individual audits within the category.

{
  "assertions": {
    "categories:performance": ["warn", {"minScore": 0.9}],
    "categories:accessibility": ["error", {"minScore": 1}]
  }
}

Levels

There are three Lighthouse CI assertion levels.

  • off - The audit result will not be checked. If an audit is not found in the assertions object, it is assumed to be off.
  • warn - The audit result will be checked, and the result will be printed to stderr, but failure will not result in a non-zero exit code.
  • error - The audit result will be checked, the result will be printed to stderr, and failure will result in a non-zero exit code.

Properties

The score, details.items.length, and numericValue properties of audit results can all be checked against configurable thresholds. Use minScore, maxLength, and maxNumericValue properties, respectively, in the options object to control the assertion.

{
  "assertions": {
    "audit-id-1": ["warn", {"maxNumericValue": 4000}],
    "audit-id-2": ["error", {"minScore": 0.8}],
    "audit-id-3": ["warn", {"maxLength": 0}]
  }
}

Aggregation Methods

When checking the results of multiple Lighthouse runs, there are multiple strategies for aggregating the results before asserting the threshold.

  • median - Use the median value from all runs.
  • optimistic - Use the value that is most likely to pass from all runs.
  • pessimistic - Use the value that is least likely to pass from all runs.
  • median-run Use the value of the run that was determined to be "most representative" of all runs based on key performance metrics. Note that this differs from median because the audit you're asserting might not be the performance metric that was used to select the median-run.

Multiple URLs / Assertion Matrix

When checking the results of runs against multiple URLs, different assertions can be made for different URL patterns.

The below example warns when FCP is above 2 seconds on all pages and warns when TTI is above 5 seconds on all secure pages whose path starts with /app. Assertion matrix configurations can be used to differentiate production from development, landing pages from single-page apps, and more.

{
  "assertMatrix": [
    {
      "matchingUrlPattern": ".*",
      "assertions": {
        "first-contentful-paint": ["warn", {"maxNumericValue": 2000}]
      }
    },
    {
      "matchingUrlPattern": "https://[^/]+/app",
      "assertions": {
        "interactive": ["warn", {"maxNumericValue": 5000}]
      }
    }
  ]
}

Presets

There are three presets available to provide a good starting point. Presets can be extended with manual assertions.

  • lighthouse:all - Asserts that every audit received a perfect score. This is extremely difficult to do. Only use as a base on very high quality, greenfield projects and lower the tresholds as needed.
  • lighthouse:recommended - Asserts that every audit outside performance received a perfect score, that no resources were flagged for performance opportunities, and warns when metric values drop below a score of 90. This is a more realistic base that disables hard failures for flaky audits.
  • lighthouse:no-pwa - lighthouse:recommended but without any of the PWA audits enabled.

The below example uses the lighthouse:no-pwa preset but disables a few audits we're not quite ready to pass yet and increases the limit on an audit with a numericValue.

{
  "preset": "lighthouse:no-pwa",
  "assertions": {
    "dom-size": ["error", {"maxNumericValue": 3000}],
    "offscreen-images": "off",
    "color-contrast": "off",
    "tap-targets": "off"
  }
}

Budgets

Instead of configuring using Lighthouse CI assertions against Lighthouse audits, a budget.json file can be used instead. This option cannot be used in conjunction with any other option.

{
  "budgetsFile": "path/from/cwd/to/budget.json"
}

If you'd like to consolidate multiple assertion configuration files and avoid multiple calls to lhci assert, you can also configure your budgets alongside your other Lighthouse CI assertions instead. Budget assertions follow the form resource-summary:<resourceType>:(size|count).

{
  "assertions": {
    "first-contentful-paint": ["warn", {"maxNumericValue": 4000}],
    "viewport": "error",
    "resource-summary:document:size": ["error", {"maxNumericValue": 400}],
    "resource-summary:font:count": ["warn", {"maxNumericValue": 1}],
    "resource-summary:third-party:count": ["warn", {"maxNumericValue": 5}]
  }
}
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