Overview

Nightwatch allows you to even define your own assertions, extending the available .assert and .verify namespaces. To do so, create a new folder (e.g. nightwatch/assertions) and start defining your own assertions inside, each in its own file.

Then specify the path to that folder in the nightwatch.json file, as the custom_assertions_path property.

nightwatch.json
{
  "custom_assertions_path" : "nightwatch/assertions"
}

Custom assertions also inherit from EventEmitter. To see some examples, check the assertions module on Github:
/nightwatch/tree/main/lib/selenium/assertions

Define a custom assertion

Assertions implement a simple interface which is shared between built-in assertions and custom ones:

nightwatch/assertions/customAssert.js
exports.assertion = function(definition, expectedText, msg) {
  
// If the custom commands operates with DOM elements, this options should be set // this.options = { // elementSelector: true // };
/** * Returns the message format which will be used to output the message in the console and also * the arguments which will be used for replace the place holders, used in the order of appearance * * The message format also takes into account whether the .not negate has been used * * @return undefined */ this.formatMessage = function() { // Use this.negate to determine if ".not" is in use // Example: const message = `Testing if the page title ${this.negate ? 'doesn't equal %s' : 'equals %s'}`;
return { message, args: [`'${expected}'`] } };
/** * Returns the expected value of the assertion which is displayed in the case of a failure * * @return {string} */ this.expected = function() { return this.negate ? `is not '${expectedText}'` : `is '${expectedText}'`; };
/** * Given the value, the condition used to evaluate if the assertion is passed * @param {*} value * @return {Boolean} */ this.evaluate = function(value) { if (typeof value != 'string') { return false; }
return value.includes(expectedText); };
/** * Called with the result object of the command to retrieve the value which is to be evaluated * * @param {Object} result * @return {*} */ this.value = function(result) { return result.value; };
/** * When defined, this method is called by the assertion runner with the command result, to determine if the * value can be retrieved successfully from the result object * * @param result * @return {boolean|*} */ this.failure = function(result) { return result === false || result && result.status === -1; };
/** * When defined, this method is called by the assertion runner with the command result to determine the actual * state of the assertion in the event of a failure * * @param {Boolean} passed * @return {string} */ this.actual = function(passed) { return passed ? `contains '${expectedText}'` : `does not contain '${expectedText}'`; };
/** * The command which is to be executed by the assertion runner; Nightwatch api is available as this.api * @param {function} callback */ this.command = function(callback) { // Example: this.api.getText(definition, callback);
setTimeout(function() { // The object containing a "value" property will be passed to the .value() method to determine the value w // which is to be evaluated (by the .evaluate() method) callback({ value: '' });
}, 1000);
};
};