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Description

Bug Report

🔎 Search Terms

undefined optional chain aliased condition narrowing

🕗 Version & Regression Information

This is the behavior in every version I tried (from 4.4 onward, when we added aliased conditions).

⏯ Playground Link

Playground Link

💻 Code

interface Foo {
    getValues(): number[];
}

declare const foo: Foo | undefined;

const vals = foo?.getValues();
if (vals) {
    foo; // Foo | undefined
}

const vals2 = foo && foo.getValues();
if (vals2) {
    foo; // Foo
}

if (foo?.getValues()) {
    foo; // Foo
}

🙁 Actual behavior

foo is potentially undefined in the first if block. The assignments fail.

(Added assignments so there's some feedback now that the playground's //^? doesn't work pre-4.6.)

🙂 Expected behavior

foo is narrowed to Foo in the first if block, matching the narrowing in the other if blocks.

Contributor guide

Research direction: Investigate how the TypeScript compiler narrows types for aliased conditions involving optional chaining. Compare the handling of `foo?.getValues()` assignment to `const vals = foo?.getValues()` and then `if (vals)` vs. direct `if (foo?.getValues())`. Focus on the control flow analysis for optional chain expressions in the binder and checker.
Tech stack: typescript
Domain: compilers
Issue type: Bug
Difficulty: 3
Estimated time: 1-2 days
Activity status: Active
Clarity: Clear
Prerequisites: TypeScript
Newbie friendliness: 30