Testing Prebid.js

Starting on 21 June 2016, all pull requests to the Prebid.js library will need to include tests with greater than 80% code coverage for any changed/added code before they can be merged into master.

For more information, see Prebid.js issue #421.

This page describes how to test code in the Prebid.js repository to help prepare your pull request.

Writing tests

When you are adding code to Prebid.js, or modifying code that isn’t covered by an existing test, test the code according to these guidelines:

  • If the module you are working on is already partially tested by a file within the test directory, add tests to that file
  • If the module does not have any tests, create a new test file
  • Group tests in a describe block
  • Test individual units of code within an it block
  • Within an it block, it may be helpful to use the “Arrange-Act-Assert” pattern
    • Arrange: set up necessary preconditions and inputs
      • e.g., creating objects, spies, etc.
    • Act: call or act on the unit under test
      • e.g., call the function you are testing with the parameters you set up
    • Assert: check that the expected results have occurred
      • e.g., use Chai assertions to check that the expected output is equal to the actual output
  • Test the public interface, not the internal implementation
  • If using global pbjs data structures in your test, take care to not completely overwrite them with your own data as that may affect other tests relying on those structures, e.g.:
    • OK: pbjs._bidsRequested.push(bidderRequestObject);
    • NOT OK: pbjs._bidsRequested = [bidderRequestObject];
  • If you need to check adloader.loadScript in a test, use a stub rather than a spy. spys trigger a network call which can result in a script error and cause unrelated unit tests to fail. stubs will let you gather information about the adloader.loadScript call without affecting external resources

  • When writing tests you may use ES2015 syntax if desired

Running tests

After checking out the Prebid.js repository and installing dev dependencies with npm install, use the following commands to run tests as you are working on code:

  • gulp test will run the test suite once (npm test is aliased to call gulp test)
  • gulp serve will run tests once and stay open, re-running tests whenever a file in the src or test directory is modified

Checking results and code coverage

Check the test results using these guidelines:

  • Look at the total number of tests run, passed, and failed (shown below the rainbow Nyan Cat in the shell window).
  • If all tests are passing, great.
  • Otherwise look for errors printed in the console for a description of the failing test.
  • You may need to iterate on your code or tests until all tests are passing.
  • Make sure existing tests still pass.
  • There is a table below the testing report that shows code coverage percentage, for each file under the src directory.
  • Each time you run tests, a code coverage report is generated in build/coverage/lcov/lcov-report/index.html.
  • This is a static HTML page that you can load in your browser.
  • On that page, navigate to the file you are testing to see which lines are being tested.
  • Red indicates that a line isn’t covered by a test.
  • Gray indicates a line that doesn’t need coverage, such as a comment or blank line.
  • Green indicates a line that is covered by tests.
  • The code you have added or modified must have greater than 80% coverage to be accepted.


Prebid.js already has lots of tests. Read them to see how Prebid.js is tested, and for inspiration:

  • Look in test/spec and its subdirectories
  • Tests for bidder adaptors are located in test/spec/adapters

A test module might have the following general structure:

// Import or require modules necessary for the test, e.g.:
import { expect } from 'chai';  // may prefer 'assert' in place of 'expect'
import adapter from 'src/adapters/<adapter>';

describe('<Adapter>', () => {

  it('<description of unit or feature being tested>', () => {
    // Arrange - set up preconditions and inputs
    // Act - call or act on the code under test
    // Assert - use chai to check that expected results have occurred

  // Add other `describe` or `it` blocks as necessary



The Prebid.js testing stack contains some of the following tools. It may be helpful to consult their documentation during the testing process.