?? and ??= behave differently when they shouldn't

TypeScript Version: 4.0.2, Nightly (2020-9-2)

Search Terms: Nullish coalescing operator Nullish never Coalescing never Nullish coalescing assignment

Code According to this post usage of ?? and ??= should be equivalent in the sample code below:

const values: { a?: string[] } = { };

delete values.a;
(values.a ?? (values.a = [])).push("hello"); // error
values.a = undefined;
(values.a ?? (values.a = [])).push("hello"); // error

delete values.a;
(values.a ??= []).push("hello");
values.a = undefined;
(values.a ??= []).push("hello");

Expected behavior: Examples with ?? and ??= should compile and behave similarly.

Actual behavior: Examples with ?? do not compile (Argument of type ‘string’ is not assignable to parameter of type ‘never’.(2345))

Playground Link: link

Related Issues: Didn’t find anything similar

[ddhorstman]

I’ve run into this bug too - is anyone working on a fix? Here’s another example of the problem in action:

type MaybeNumber = number | undefined;
const a: MaybeNumber = undefined;
const b = 5;
let x: MaybeNumber, y: MaybeNumber, z: MaybeNumber;

// Chained nullish coalescing
x = (x ?? a ?? b);
console.log(x + 1); // 6

// Single nullish assignment
y ??= b;
console.log(y + 1); // 6

// Nullish assignment into nullish coalescing
z ??= a ?? b;
console.log(z + 1); // Object is possibly 'undefined'

playground link

As I understand it, x and z should behave identically. Is this difference in behavior somehow intentional?

[stevenwdv]

I found a different variant of the bug (posted first as a comment to #40494).

Take the following code snippet, where there is a bug (?? should have been ||).

let elem!: Element;

let selector1 =
    (elem.hasAttribute('name') && `[name='${elem.getAttribute('name')!}']`)
    ?? elem.tagName;
const s1: string = selector1;  // Correctly reports that false is not assignable to string

let selector2;
selector2 = elem.hasAttribute('name') && `[name='${elem.getAttribute('name')!}']`;
selector2 ??= elem.tagName;
const s2: string = selector2;  // No error as selector2 has been incorrectly narrowed to string, where there should be!!

It seems as though ??= incorrectly works identical to ||=. This is not a regression, it was apparently always there.

Playground link