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noNonNullAssertion (since v1.0.0)

Diagnostic Category: lint/style/noNonNullAssertion

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 Examples

Invalid

Section titled 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 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 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

Section titled Valid
interface Example {
  property?: string;
}


declare const example: Example;
const includesBaz = foo.property?.includes('baz') ?? false;
Section titled Related links