main.js

const { app, BrowserWindow } = require( "electron" )

app.whenReady().then(()=>{

	let main = new BrowserWindow({
		"height" : 600,
		"width" : 800
	})

	main.loadURL( "https://www.youtube.com" )
	main.webContents.on( "did-finish-load", ()=>{

		let iframe = main.webContents.mainFrame.frames[ 0 ]
		let injection = "new Promise(( resolve )=>{ resolve('hello') })"
		let handler = ( result ) => { console.log( result ) }

		//this code does not evaluate the same:
		iframe.executeJavaScript( injection ).then( handler ) //returns "{}"
		main.webContents.executeJavaScript( injection ).then( handler ) //returns "hello"

	})
})

package.json

{
  "main": "main.js",
  "scripts": {
    "start": "electron ."
  },
  "devDependencies": {
    "electron": "17.0.0"
  }
}

Just to outline the potentially obvious...

This gist only works and reproduces the bug if the URL on line 10 meets the following criteria...:

• has an iframe
• iframe exists* prior to the "did-finish-load" event

^{* - This is based on my personal understanding of the executeJavaScript method. I believe it can be executed without having anything loaded in the iframe}