cypress/guides/error-handling.md

Error Handling in Cypress

Clear, consistent, errors are one of the important parts of the Cypress experience. When something goes wrong, there should be clear, actionable feedback for the user about exactly what went wrong, where it went wrong, and next steps on how to fix.

@packages/errors

All error related logic for the server should be added to @packages/errors. This logic has been separated out from the @packages/server to enable strict type checking & use in other packages we have added in the 10.0-release branch.

Summary of the Errors package:

• errors.ts: A key/value mapping of known errors to functions returning "ErrorTemplates", described below, also includes/re-exports several helper utilities:
  • get / getError: builds & retrieves the error as a CypressError, should be the main way we retrieve errors throughout Cypress. Aliased as errors.get for existing use in the server package
  • throw / throwErr: Get & throw the error, so we can spy/stub it in a test. Aliased as errors.throw for existing use in the server package
  • logWarning: Logs the error as a warning to the console, aliased as errors.log for existing use in the server package
• errTemplate.ts: Tagged template literal formatting the error as described below
• stackUtils.ts: Utilities for working with a stack trace, extended by the driver package

errTemplate

The errTemplate is a tagged template literal. It allows us to maintain consistent behavior & formatting in our error messages, when we see a variable, we format it depending on the target environment.

The error message returns a message that defaults to being formatted for the terminal, and has a forBrowser method which returns the error message where the variables are wrapped in backticks '`' for Markdown display in the browser.

Return Value of errTemplate (ErrTemplateResult):

{
  // Will always exist, this is the terminal-formatted error message
  message: string, 
  // Will always exist, this is the browser-formatted error message
  messageMarkdown: string, 
  details?: string, // Exists if there is `details()` call in the errTemplate
  originalError?: ErrorLike // Exists if an error was passed into the `details()`
}

Example:

CANNOT_TRASH_ASSETS: (arg1: string) => {
  return errTemplate`\
      Warning: We failed to trash the existing run results.

      This error will not alter the exit code.

      ${details(arg1)}`
},  

In this case, arg1 will be highlighted in yellow when printed to the terminal.

PLUGINS_FILE_ERROR: (arg1: string, arg2: Error) => {
  return errTemplate`\
      The plugins file is missing or invalid.

      Your \`pluginsFile\` is set to ${arg1}, but either the file is missing, it contains a syntax error, or threw an error when required. The \`pluginsFile\` must be a \`.js\`, \`.ts\`, or \`.coffee\` file.

      Or you might have renamed the extension of your \`pluginsFile\`. If that's the case, restart the test runner.

      Please fix this, or set \`pluginsFile\` to \`false\` if a plugins file is not necessary for your project.

      ${details(arg2)}
    `
},

arg1 will be highlighted in blue for the terminal, and wrapped in backticks when called with forBrowser. Details will be printed in yellow as a stack trace when printed to the terminal, or shown as a stack-trace in the browser.

Error Wrapping

Any time we know about an edge case that is an error, we should define an error in errors.ts. This error should be retrieved by getError, which converts it to a CypressError.

The CypressError is an Error containing the message returned from the errTemplate. The stack is set to that of the originalError if it exists (i.e. the error object passed into details), otherwise it's the stack from where the getError / throwError is called.

The CypressError has an isCypressErr prop which we use as a duck-type guard against exiting the process when logging exceptions. It also maintains a reference to the originalError if it exists.

Child-Process Errors

All errors that occur in a child process spawned by Cypress should be sent over the ipc bridge using util.serializeError.

This ensures the name, message, stack, and any other relevant details are preserved and can be handled by the standard process of Cypress' error standardization / wrapping.