Manager: Could you create the UI for the new feature? The client wants to test it. We need it in 3 days.

*1 week later*

Client: IT DOESNT WORK

Me: This is just a visual demo... but everything will work when we realse the feature.

Client: okay but can I see what it will do?

Of course you can! Just wait until we relase it!

*2 weeks later*

Manager: What are you doing?

Me: Working on the UI for the new feature.

Manager: Wait, hadn't you already done it for the demo?

Me: That UI didn't really work. It was basically a bunch of HTML, without reactivity or abstraction or any functionality.

Manager: Okay, how much where you able to re-use?

Me: almost nothing.

Manager: So... you wasted those 3 days?

Oh so I'm the one who wasted 3 days.
Me: Kinda, yeah

Manager: Why couldn't you have done this when I asked you to do the UI?

You can't expect good quality code in 3 days. Pls stop wasting it on demos.