Playground auto-run can hang the page when running unfinished code

[nasso]

Today I was playing with nested $effects in the playground to understand them better. At some point, I needed to write a for loop:

for (const thing of things) {
  ...
}

Amazing! Actually, not amazing. Turns out I needed the index. A clever guy would have reached for Array.prototype.entries(), but I'm not a clever guy, so I immediately went for the good old counter. This is roughly how I updated my code:

1. for (|) {
2. for (let |) {
3. for (let i = |) {
4. for (let i = 0; |) {
5. for (let i = 0; i < |) {
6. for (let i = 0; i < things.|) {
7. for (let i = 0; i < things.length|) {
8. for (let i = 0; i < things.length; i|) {

At this point, I wasn't done typing. I only had ++ left. But I couldn't type! In fact, I couldn't do anything anymore on the entire page. And then it hit me. It ran my unfinished for loop! And without the increment, it wasn't gonna stop any time soon.

I see three solutions to this problem:

1. Add a toggle in the UI to disable auto-run – but not everyone will think about this before it's too late.
2. Solve the halting problem to decide if the code is safe to run – this might have applications outside of the Svelte Playground.
3. Stop the script after some time – will break scripts doing a lot of things in a blocking manner on the main thread (but maybe they deserve it)