Local function variables are suggested within its default arguments

[Andarist]

🔎 Search Terms

autocomplete lsp suggestion default parameter

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABFApgZygRgBQENEC8iA9AFSnECUiA3gFCKIBuuATogBZwwYoAmDRABsUURAFsQUXACMRgiAgyJWKXHwRCAnoUSY6AXyA

💻 Code

function test1(a = /**/) {
  var hoisted
  let mutable
  const readonly = 1
}

🙁 Actual behavior

All 3 are suggested at the marker

🙂 Expected behavior

None of them should be suggested as they can't be used there

Additional information about the issue

No response

[TulsiLukhi1]

Hi,
At first, I struggled to understand the issue, but after thinking it through, I finally grasped what you were explaining. That's why I’m attaching a reference image to illustrate what you meant so it can be helpful for someone referring to this in future.

When defining a default parameter in a function, TypeScript currently suggests variables declared inside the function body. However, this shouldn't happen because default parameters are assigned at the time of function call, and variables inside the function body are not yet initialized at that point.

Since there's no way to access function-scoped variables when assigning default parameter values, TypeScript should not include them in autocomplete suggestions.

Proposed Fix
To resolve this, changes need to be made in the TypeScript codebase at:https://github.com/microsoft/TypeScript/blob/main/src/services/completions.ts

Specifically, in the getCompletionData function, we should add a check to filter out function-scoped variables when inside a default parameter.

This is partial code I have attached to understand what can be done!

symbols = symbols.filter(symbol => {
        if (symbol.flags & SymbolFlags.Global) {
            return true; // ✅ Allow global variables
        }

        if (symbol.flags & SymbolFlags.FunctionScopedVariable) {
            return isDeclaredBeforeFunctionBody(symbol, functionDeclaration);
        }

        return false; // ❌ Block local function variables
    });

We need to implement isDeclaredBeforeFunctionBody to check whether a symbol was declared before the function body.

I believe this fix should work!

I would love to contribute to this issue as I have already identified the necessary changes.
Could I be assigned to work on this?

[mkantor]

@TulsiLukhi1 no need to be "assigned", you can just start working on this. See this part of CONTRIBUTING.md for details (you might want to read the whole document if this is your first time contributing).