# Inexplicably broken project (sim error: failed cast on undefined)
**URL:** https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857
**Category:** Help
**Created:** [May 21, 2024, 10:43pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857 "2024-05-21T22:43:52Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Sarge](https://forum.makecode.com/u/Sarge)
#### Post date: [May 21, 2024, 10:43pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857/1 "2024-05-21T22:43:52Z")
A project I’m working on inexplicably crashes when I run it. The cause of the error seems to be another one of my extensions which i’m using in my project. However, when I copied my entire code into that extension’s project (in a separate file) and ran the exact same code it worked perfectly. I have no idea why this is happening. Please help (@richard? @AlexK?)
You can get the project at [https://github.com/sargedev/betterarrays](https://github.com/sargedev/betterarrays)
Didn’t want to leak it but oh well, most people know what I’ve been working on anyway.
This is a project I’ve been working on extensively (no pun intended) for the past few weeks and now that I’m this close to release it breaks, so all help is appreciated.
---
### Author:  [@richard](https://forum.makecode.com/u/richard)
#### Post date: [May 21, 2024, 11:00pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857/2 "2024-05-21T23:00:32Z")
@Sarge this is being caused by a quirk in how we run user code.
When MakeCode compiles a project, it always makes sure that `main.ts` is last to execute. If you declare a variable inside `main.ts` and then reference it in a function that runs before `main.ts`, the variable will be undefined. In this case, that variable is the `NON_INTEGER_VALUE` format string you declare at the top of your file.
Luckily, the fix is super easy. Add another file named something other than `main.ts` to your project and move everything into that instead. Then just leave `main.ts` blank.
---
### Author:  [@richard](https://forum.makecode.com/u/richard)
#### Post date: [May 21, 2024, 11:05pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857/3 "2024-05-21T23:05:59Z")
Also worth noting that `main.ts` is the only special file that works this way. You can control the order in which other files are processed by rearranging them inside the files array in `pxt.json`.
Still, you don’t want to delete `main.ts` entirely since that might cause some bugs (or it might be fine, but I’m willing to bet there are some parts of makecode that assume a `main.ts` file is always present).
---
### Author:  [@Sarge](https://forum.makecode.com/u/Sarge)
#### Post date: [May 22, 2024, 4:24pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857/4 "2024-05-22T16:24:54Z")
@richard So, I fixed that, and development went smoothly, until I ran into another wonky issue while formatting. I thought it was just a bug with my extension, so just like last time I all the necessary code there, and would you know it - it worked perfectly.
~~Never have I been this mad my code actually works 😂~~
Anyway, not only does it work in another project, pasting the code at the bottom of `api.ts` (which is now my main file) also yields the intended results.
In `testFill` (`test.ts`), this snippet of code executes
```js
arrays.fill(["a", "b", "c"], "a", 4, 0);
```
which is supposed to throw `Index (4) must be in list range (0, 3)`
It however throws `Index (2) must be in list range (0, 2)` (wrong values)
Like I said, in the api file it throws it with the correct values. I am stumped, why could this be happening. Any help is appreciated
---
### Author:  [@richard](https://forum.makecode.com/u/richard)
#### Post date: [May 22, 2024, 4:40pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857/5 "2024-05-22T16:40:28Z")
well i can tell you that one of your tests is interfering with the other, though I haven’t debugged why. it’s one of the tests from the `toSwapped` test function.
you can reproduce the exception with this:
```auto
new tests.AssertRaises(
() => arrays.toSwapped(["a", "b"], 2, 0),
"Index (2) must be in list range (0, 2)"
)
new tests.AssertRaises(
() => arrays.fill(["a", "b", "c"], "a", 0, 4),
"Index (4) must be in list range (0, 3)"
)
```
only reproduces if the `toSwapped` test case runs before the other
---
### Author:  [@richard](https://forum.makecode.com/u/richard)
#### Post date: [May 22, 2024, 7:18pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857/6 "2024-05-22T19:18:45Z")
alright, i went ahead and looked into it. pretty sure the error is in your text formatting extension. in this method:
```auto
format(params: string[]): string {
let index = 0;
for (let i = 0; i < this.parsed.length; i++) {
if (this.parsed[i] === null) {
if (index >= params.length) throw `Expected ${this.paramNum} param(s) (got ${params.length})`;
this.parsed[i] = params[index];
index += 1;
}
}
return this.parsed.join("");
}
```
not sure why you’re modifying `this.parsed` but i’m betting that the state is getting carried over between tests. try building up a string as you go instead of modifying the array and calling join
---
### Author:  [@Sarge](https://forum.makecode.com/u/Sarge)
#### Post date: [May 22, 2024, 7:25pm UTC](https://forum.makecode.com/t/inexplicably-broken-project-sim-error-failed-cast-on-undefined/28857/7 "2024-05-22T19:25:27Z")
Yup, figured that out just a moment ago! Don’t know what the thought process behind that was, I probably wrote that at 2AM or something close 😅
Fixed now by just adding a result variable which builds up over time.
Thank you very much, you’ve helped a ton in diagnosing issues (both makecode, and my code related, lol)! Also, I should probably start commenting stuff. No idea how you managed to find your way in that mess