false positive error after null checking with typed boolean variable instead non typed variable #56369

🔎 Search Terms

false positive error after null checking with typed boolean variable instead non typed variable
strictNullChecks false positive error

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about 2.0.3 to 5.4.0-dev.20231112
• I was unable to test this on prior versions because of insufficient knowledge with older typescript versions

⏯ Playground Link

TypeScript: TS Playground

💻 Code

Example code

Common base

The function simply returns a type or a null:

tsconfig.json

{
    "compilerOptions": {
        "strictNullChecks": true
    }
}

index.ts

type Example = {
    field: string
}

function test(): Example | null {
    return { field: "Hello, world!" };
}
// ...

Code with the Issue

The following code doesn't compile. It can't recognize the null checking of the variable

index.ts

// ...
const variable = test();
// Can be literally any type, any, boolean, Boolean
const isVariableNull: any = !variable;
if (!isVariableNull) {
    // Property 'field' does not exist on type 'Example | null'.(2339)
    const { field } = variable;
}

Equivalent working code

The following code is equivalent to the previous one and correctly compiles.

index.ts

// ...
const variable = test();
// Mustn't be typed
const isVariableNull = !variable;
if (!isVariableNull) {
    // Property 'field' can be normally used here
    const { field } = variable;
}

🙁 Actual behavior

TypeScript can't recognize null checking of variables when the value is assigned to a typed variable and later used to do the null checking

🙂 Expected behavior

TypeScript should recognize the null checking and compile the code normally (Like it's equivalent snippet)

Additional information about the issue

No response