🔎 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