Extending my TypeScript auto-fix CLI to handle .filter(Boolean) narrowing issues

i-am-killvish — Mon, 04 May 2026 16:29:23 +0000

A few days ago I shared a small CLI experiment called fixmyfile that automatically fixes repetitive TypeScript errors using compiler diagnostics and AST transformations.

While testing it more, I kept running into another frustrating TypeScript pattern:

const users = data.filter(Boolean);

users.map((u) => u.name);

Even after filtering values, TypeScript can still complain that:

'u' is possibly undefined

Logically the values are already filtered, but TypeScript narrowing does not always behave the way developers expect.

So I started experimenting with AST-based transformations for these cases too.

Now the CLI automatically converts:

const users = data.filter(Boolean);

into:

const users = data.filter(
  (x): x is NonNullable<typeof x> => Boolean(x)
);

which preserves narrowing correctly and removes the repeated "possibly undefined" friction afterward.

What’s been interesting while building this project is realizing how many TypeScript frustrations are not necessarily difficult problems, but repetitive workflow interruptions developers hit every day.

Still very experimental, but AST transformations are turning out to be much more powerful than simple text replacements.

Current fixes supported:

Missing function arguments (TS2554)
Basic type mismatch fixes (TS2322)
Undefined references (TS2304)
.filter(Boolean) narrowing transformations

GitHub:
https://github.com/i-am-killvish/fixmyfile

Would love feedback on:

other repetitive TS friction points
weird narrowing cases
compiler issues developers repeatedly fight with

I built a CLI that fixes TypeScript errors automatically

i-am-killvish — Sat, 25 Apr 2026 01:20:24 +0000

Fixing TypeScript errors like:

“Expected 1 arguments, but got 0 (TS2554)”

gets repetitive really fast.

So I built a small CLI tool called fixmyfile that automatically fixes some common TypeScript errors using the compiler API and AST transformations.

⚡ Demo

Fixing a TypeScript error automatically:

🧠 What it does

The tool:

scans a .ts file
detects TypeScript compiler errors
applies automatic fixes
repeats until no more changes are possible

⚡ Example

❌ Before
function greet(name: string) {
console.log(name);
}

greet(); // missing args
✅ After
greet("text");

🖥 Try it

You can run it directly using:

npx fixmyfile yourfile.ts

🧩 Currently supports

Missing function arguments (TS2554)
Basic type mismatch fixes (TS2322)
Undefined references (TS2304)

⚠️ Note

This is still experimental and focuses on common, repetitive fixes.
Always review the output before committing changes.

💬 Feedback

Would love to know:

Did it work on your code?
What errors should it support next?
Where did it break?

👉 https://github.com/i-am-killvish/fixmyfile

🚀 Why I built this

I kept running into the same TypeScript errors during development and realized many of them are predictable enough to automate.

This is an early attempt at making that process faster.

If this sounds useful, give it a try and let me know what you think 🙂

Forem: i-am-killvish