@typescript-eslint/no-non-null-asserted-nullish-coalescing

?? gives a fallback for when the left side is null or undefined. A ! on that side says it never is. If that's true, the fallback is dead code. If it isn't, the ! is wrong. ?? handles both cases without it, so remove the !.

A ! on the right side is allowed.

Configure

Off by default. To turn it on:

[lint.rules]
"@typescript-eslint/no-non-null-asserted-nullish-coalescing" = "error"

Examples

Reported:

declare const settings: { theme?: string };
declare function findUser(id: string): { name: string } | null;

const theme = settings.theme! ?? "light";
const user = findUser("42")! ?? { name: "guest" };

Not reported:

declare const settings: { theme?: string };
declare function findUser(id: string): { name: string } | null;

const theme = settings.theme ?? "light";
const user = findUser("42") ?? { name: "guest" };

Here the ! is what lets you read label before it's assigned, so it's allowed:

let label: string;

const text = label! ?? "untitled";