Is it possible to inject Javascript code to DOM before/during a Cypress test and run the injected code?

[kaphula]

Hello,

My use-case is that I'd like to write Javascript code (that uses my custom library) inside the Cypress tests. The way I do this normally is that I have the index.html file where I just declare a <script> block and insert the code I want to run inside. The index.html file that Cypress loads can execute the loading of my library and the custom code that uses this library fine when that code already exists in the index.html, but can I manipulate the code that will be executed inside the Cypress tests?