Verify Response - Assertions

When testing an API, it is usually not enough to verify that all our HTTP requests completed successfully. Often, it is necessary to make sure that we got the correct response from the server by examining the response status, body or headers.

With Loadmill, this is made easy by using Assertions. Assertions are used in conjunction with parameters to do just that: examine the server's response and assert its correctness.

You may have an arbitrary number of assertions executed after each successful request. If an assertion fails, the next request will not be executed and the test scenario will be marked as failed, but the subsequent assertions for the current request will be executed nonetheless.

Assertion Types

The target of an assertion is always a parameter value. You may use built-in parameters, default parameters or any parameter extracted from the current or previous requests in the current scenario as the target.

There are several types of assertions:

  1. Is True (Not Empty) - the target is a JavaScript truthy value. This includes non-empty strings and numbers.

  2. Is False (Empty) - the target is the JavaScript falsy value or empty.

  3. Equals - the target is equal to the given expression. The equality check is case sensitive.

  4. Contains - the target contains the given expression. The containment check is case sensitive.

  5. Matches JS RegExp - the target matches the given regular expression.

You may embed parameters in any assertion expression. These parameters will be evaluated right before the assertion is executed.


Keep in mind that all parameter values are textual, i.e. a parameter has no type such as Number or Array that we know from common programming languages.

This is important in order to avoid confusion when using parameter extractors such as JSONPath. For example, consider the following scenario:

  1. Extract the value for books via the JSONPath query student.books on

    "student": {
    "books": []
  2. Assert books is Not Empty.

You may expect this assertion to fail but, in fact, it will succeedd. This is because the parameter books is evaluated to [] and therefore constitues a non-empty string. One possible way to correct this is to use a RegExp assertion on the books parameter instead: \[[^\s]+\].