@typescript-eslint/no-non-null-assertion

value! tells TypeScript that value isn't null or undefined. Nothing checks that when the code runs. If you're wrong, or the code around it changes, value!.name throws a TypeError, which is the error strict null checks are meant to prevent.

Check the value, or use ?. or ??, which do their work at runtime.

Configure#

Off by default. To turn it on:

[lint.rules]
"@typescript-eslint/no-non-null-assertion" = "error" # or "warn"

Examples#

const users = new Map<string, { name: string }>();

function greet(id: string) {
  const form = document.querySelector("form")!;
  form.title = users.get(id)!.name;
}

const users = new Map<string, { name: string }>();

function greet(id: string) {
  const form = document.querySelector("form");
  if (!form) {
    throw new Error("The page has no form");
  }
  form.title = users.get(id)?.name ?? "guest";
}

When not to use it#

Sometimes you know more than the type checker, for example right after map.has(key). If your code has many places like that and you trust them, this rule is noise.