noNonNullAssertion

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TypeScript and TSX

Summary

Rule available since: v1.0.0
Diagnostic Category: lint/style/noNonNullAssertion
This rule is recommended, which means is enabled by default.
This rule has an unsafe fix.
The default severity of this rule is warning.
Sources:
- Same as @typescript-eslint/no-non-null-assertion

How to configure

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

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

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interface Example {
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  property?: string;
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}
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declare const foo: Example;
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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 │

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(b!! as number) = "test";

code-block.ts:1:2 lint/style/noNonNullAssertion ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Forbidden non-null assertion.
  
  > 1 │ (b!! as number) = “test”;
      │  ^^^
    2 │

Valid

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