noNonNullAssertion

Diagnostic Category: lint/style/noNonNullAssertion

TypeScript and TSX

Since: v1.0.0

Sources:

Same as: @typescript-eslint/no-non-null-assertion

Description

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 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

interface Example {
  property?: string;
}

declare const foo: Example;
const includesBaz = foo.property?.includes('baz') ?? false;

How to configure

{
  "linter": {
    "rules": {
      "style": {
        "noNonNullAssertion": "error"
      }
    }
  }
}