# Execute JavaScript

Run arbitrary JavaScript inside Endtest Web Tests. Read page state, call internal functions, manipulate the DOM, and capture results into variables.

Part of the Endtest documentation (Web Tests). Canonical URL: https://endtest.io/docs/web-tests/execute-javascript

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## Introduction

There are certain scenarios where you might need to execute some JavaScript code.

For example, let's say you want to generate a complex random pattern that you cannot obtain by using our `Set Variable` action and by concatenating variables and strings.

You can execute JavaScript code in your **Web Tests** by using an `Execute JavaScript` step.

The JavaScript variables and the Endtest variables are on different layers, but you can exchange values between them:

- You can transfer a value **from** a JavaScript variable **to** an Endtest variable by using the `Extract Value from JS variable` option in the `Set Variable` action.  
- You can transfer a value **from** an Endtest variable **to** a JavaScript variable by using the `Transfer to JavaScript` action.

 You can find more details about the `Set Variable` action and variable concatenation in the [Variables](https://endtest.io/docs/advanced/how-to-use-variables) chapter. 

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## Steps

1. Add an `Execute JavaScript` step.  
2. Write your JavaScript code in our JavaScript editor.  
3. Save the test.  
4. Execute the test.

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## Mandatory Guidelines

1. **Do not** use the `var`, `let`, or `const` keywords when declaring variables and constants in your JavaScript code.  
2. **Do not** add comments in your JavaScript code.  
3. Test your JavaScript code in your own browser console to make sure that it works as expected.  
4. Avoid using `Execute JavaScript` to click elements or write text in inputs.  
   - Using JavaScript to perform such actions does not mimic a real user and your web application might behave differently.  
5. The JavaScript code is executed by the specific WebDriver for each browser.  
   - This means it might not behave the same way in all browsers, and you might need to adapt it.  
   - For example, if you want your code to run correctly in Firefox, you must use `window.` before each variable.  
6. If the execution of the JavaScript throws any errors, you will see them in the output of that step.

[Video](https://endtest-videos.s3-us-west-2.amazonaws.com/documentation/endtest_execute_js.mp4)

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## External JavaScript Libraries

You can also import an external JavaScript library in an `Execute JavaScript` step.

For example, here's how you might import the `Chance.js` library:

```javascript
s = document.createElement("script");
s.type = "text/javascript";
s.src = "https://chancejs.com/chance.min.js";
document.getElementsByTagName("head")[0].appendChild(s);
```
And this is how you would call the `phone()` method from that library:

```javascript
phone = chance.phone();
```

 The methods from the imported JavaScript library cannot be called from the same `Execute JavaScript` step. A new `Execute JavaScript` step should be added for that. 

As mentioned at the beginning of this chapter, you can transfer a value from a JavaScript variable to an Endtest variable by using a `Transfer to JavaScript` step.