Hi,
By mainWindow you mean in an Electron project from the main.js file? If that is the case, mainWindow is not exposed as a global variable and it should stay that way for security reasons. You can however try to send messages through IPC to your main.js.
Example on this messaging mechanism from your previous question.
Normally if you have a JS file that you include in your project.html and that JS file has global variables/objects, then you can use those global variables/objects from an asm block. Do you experience issues with this?