noNonNullAssertion (since v11.0.0)
This rule is recommended by Rome.
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.
Examples
Invalid
interface Example {
property?: string;
}
declare const example: Example;
const includesBaz = foo.property!.includes('baz');style/noNonNullAssertion.js:5:21 lint/style/noNonNullAssertion FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━
✖ Forbidden non-null assertion.
3 │ }
4 │ declare const example: Example;
> 5 │ const includesBaz = foo.property!.includes('baz');
│ ^^^^^^^^^^^^^
6 │
ℹ Suggested fix: Replace with optional chain operator ?. This operator includes runtime checks, so it is safer than the compile-only non-null assertion operator
3 3 │ }
4 4 │ declare const example: Example;
5 │ - const·includesBaz·=·foo.property!.includes('baz');
5 │ + const·includesBaz·=·foo.property?.includes('baz');
6 6 │
(b!! as number) = "test";style/noNonNullAssertion.js:1:2 lint/style/noNonNullAssertion ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
✖ Forbidden non-null assertion.
> 1 │ (b!! as number) = "test";
│ ^^^
2 │
Valid
interface Example {
property?: string;
}
declare const example: Example;
const includesBaz = foo.property?.includes('baz') ?? false;