Bug Report

🔎 Search Terms

assignment operator, noUncheckedIndexedAccess

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

// @exactOptionalPropertyTypes

declare const xx: { prop?: number };

xx.prop;
//  ^?
// typeof xx.prop === number | undefined ✅

function aa() {
xx.prop.toFixed();
//  ^?
// typeof xx.prop === number | undefined ✅
}

function bb() {
xx.prop += 1;
//  ^?
// typeof xx.prop === number | undefined ✅
}

function cc() {
xx.prop ??= 1;
//  ^?
// typeof xx.prop === number ❌
}

function dd() {
xx.prop ?? (xx.prop = 1);
//  ^?
// typeof xx.prop === number | undefined ✅
}

function ee() {
xx.prop = 1;
//  ^?
// typeof xx.prop === number ❌
}

🙁 Actual behavior

TS reports the type of xx.prop to be just number when using the ??= assignment operator.
This is incorrect - xx.prop might be undefined until AFTER the assignment and also is different behaviour compared to other assignment operators and different to how the expanded assignment is typed.

🙂 Expected behavior

TS reports the type of xx.prop to be number | undefined before the assignment.

Additional info

In addition to the above - this behaviour makes it difficult for us to write lint rules because when we attempt to check if the LHS of the assignment is undefined, the type system reports that it is not. Example issue typescript-eslint/typescript-eslint#6635