Executing JavaScript in Tests

Ghost Inspector gives you the ability to execute custom JavaScript within the browser during your test runs. You can use this to feature to simply execute arbitrary code, return a true/false value as an assertion, or extract data into a variable.

Table of Contents

  • Execute JavaScript

  • JavaScript returns true

  • Extract from JavaScript

  • Asynchronous JavaScript Execution

Execute JavaScript

Using the Execute JavaScript operation, we can execute JavaScript code that interacts with the browser test’s current webpage. We can even output things to the browser’s console, which gets recorded with the test. This example takes the content of the #header element and outputs it with console.log().

Execute JavaScript

JavaScript returns true

Also available is a JavaScript returns true assertion. This assertion type allows you to execute custom JavaScript code and return true or false (marking the assertion as passed or failed, respectively). In this example, we get the computed style for the #header element and assert that the margin-bottom property is set to 30px.

JavaScript returns true

Extract from JavaScript

Lastly is the Extract from JavaScript operation. This command allows you to return a value from custom client-side JavaScript and store it in a variable for use in later steps.

Extract from JavaScript

You can also use this functionality, along with JavaScript’s Date functions to create and use custom date strings in your test. This example will return the current date in YYYY-MM-DD format.

Extract from JavaScript

Asynchronous JavaScript Execution

Each of our JavaScript execution steps also supports asynchronous execution through the use of the following Promise template:

return new Promise(function (resolve, reject) {
  // your code here
  setTimeout(function () {
  }, 1000)

In this example we are executing an asynchronous setTimeout function and resolving the Promise after 1 second.

JavaScript returns true Example

The following simple example will pass a JavaScript returns true step:

return new Promise(function (resolve, reject) {

Any rejection or error within the Promise will fail the step.

Extract from JavaScript XHR Example

We can use the following example to extract the value within an Extract from JavaScript step using an AJAX request:

return new Promise(function (resolve, reject) {
  var oReq = new XMLHttpRequest()
  oReq.addEventListener('load', function () {
  oReq.addEventListener('error', function (error) {
  oReq.open('GET', 'https://jsonplaceholder.typicode.com/todos/1')

Here we've added both a load and error listener to an XMLHttpRequest object and sent off a GET request to our API. When the response is returned, we resolve() the promise. The returned value will be the JSON text of the response.

We can then inject resulting the JSON into a subsequent JavaScript step, in this case we'll use the response in combination with a JavaScript returns true step to assert our response is as expected:

Extract JSON with async JavaScript example

Because the response is JSON proper, the value myJSON is evaluated as plain JavaScript in the second step and we can use it directly in our script as const myObject = {{myJson}}:

Resulting JavaScript step execution with JSON

Notes and Limitations for Asynchronous JavaScript

The following behaviours apply to all asynchronous JavaScript steps:

  • Async scripts must start with return new Promise( and end with }).
  • The Promise must resolve() within the Element Timeout setting of the test or the step will fail.
  • Any reject()ed promises will result in a failing step.
  • Any uncaught Error thrown within the script will result in a failing step.