no-non-null-assertion

Disallow non-null assertions using the ! postfix operator.

TypeScript's ! non-null assertion operator asserts to the type system that an expression is non-nullable, as in not null or undefined. Using assertions to tell the type system new information is often a sign that code is not fully type-safe. It's generally better to structure program logic so that TypeScript understands when values may be nullable.

Invalid:

interface Example {
  property?: string;
}
declare const example: Example;

const includes = example.property!.includes("foo");

Valid:

interface Example {
  property?: string;
}
declare const example: Example;

const includes = example.property?.includes("foo") ?? false;