noNonNullAssertion
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Diagnostic Category: lint/style/noNonNullAssertion
Since: v1.0.0
Sources:
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
Section titled ExamplesInvalid
Section titled Invalidinterface Example { property?: string;}declare const foo: Example;const includesBaz = foo.property!.includes('baz');code-block.ts:5:21 lint/style/noNonNullAssertion FIXABLE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
✖ Forbidden non-null assertion.
3 │ }
4 │ declare const foo: Example;
> 5 │ const includesBaz = foo.property!.includes(‘baz’);
│ ^^^^^^^^^^^^^
6 │
ℹ Unsafe 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 foo: Example;
5 │ - const·includesBaz·=·foo.property!.includes(‘baz’);
5 │ + const·includesBaz·=·foo.property?.includes(‘baz’);
6 6 │
(b!! as number) = "test";code-block.ts:1:2 lint/style/noNonNullAssertion ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
✖ Forbidden non-null assertion.
> 1 │ (b!! as number) = “test”;
│ ^^^
2 │
Valid
Section titled Validinterface Example { property?: string;}
declare const foo: Example;const includesBaz = foo.property?.includes('baz') ?? false;