You've misunderstood the GP's question. If you read the other answers you might understand what he's asking. Hence exactly why they're all talking about iframes

You've misunderstood the GP's question. If you read the other answers you might understand what he's asking. Hence exactly why they're all talking about iframes. You used to be able to do it quite easily, but it meant people could essentially impersonate the user if you got them to execute some javascript. So having a code editor would be a recipe for account hijacking. So gradually browsers locked it all down. Long gone are the days of just doing 'eval()'. In the 2000s I worked on code where we actually did that! Ah, the days of getting away with massive security holes that no-one even knew how to exploit.