• formatString - the format of an error message that might be throw

The f() method of assertions formats any error messages associated with a group of assertions. The method can be called at any point in the group to set the error message format. If validate() or remains() is called, a new group is created and the default format is restored.

Within the formatString two replacements are done to provide better error messages. The first replaces %s with the scope of the interactor. If an interactor does not have a scope associated with it the interactor's class name is used instead. The second replacement is %e which will become the failed assertion's error message.

For example, the below code is how the click() action is implemented:

function click(selector) {
  return scoped(selector)
  // perform clickable validation
    .assert.f('Failed to click %s: %e')
  // invoke the native DOM method
    .do(element => {;

In the example above, the thrown error message might look like:
Failed to click "button": is disabled.