useUnifiedTypeSignatures

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TypeScript and TSX

Summary

Rule available since: v2.1.0
Diagnostic Category: lint/style/useUnifiedTypeSignatures
This rule has an unsafe fix.
The default severity of this rule is information.
Sources:
- Same as @typescript-eslint/unified-signatures

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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        "useUnifiedTypeSignatures": "error"
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      }
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    }
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  }
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}

Description

Disallow overload signatures that can be unified into a single signature.

Overload signatures that can be merged into a single signature are redundant and should be avoided. This rule helps simplify function signatures by combining overloads by making parameters optional and/or using type unions.

Examples

Invalid

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function f(a: number): void;
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function f(a: string): void;

code-block.ts:1:1 lint/style/useUnifiedTypeSignatures  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Overload signatures are hard to read and maintain.
  
  > 1 │ function f(a: number): void;
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ function f(a: string): void;
    3 │ 
  
  ℹ Unsafe fix: Combine overloads using a type union.
  
    1   │ - function·f(a:·number):·void;
    2   │ - function·f(a:·string):·void;
      1 │ + 
      2 │ + function·f(a:·string·|·number):·void;
    3 3 │

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interface I {
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    a(): void;
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    a(x: number): void;
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}

code-block.ts:2:5 lint/style/useUnifiedTypeSignatures  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Overload signatures are hard to read and maintain.
  
    1 │ interface I {
  > 2 │     a(): void;
      │     ^^^^^^^^^^
    3 │     a(x: number): void;
    4 │ }
  
  ℹ Unsafe fix: Combine overloads by making parameters optional.
  
    1 1 │   interface I {
    2   │ - ····a():·void;
    3   │ - ····a(x:·number):·void;
      2 │ + ····a(x?:·number):·void;
    4 3 │   }
    5 4 │

Valid

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function f(a: number | string): void {}

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interface I {
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    a(x?: number): void;
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}

Different return types cannot be merged:

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interface I {
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    f(): void;
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    f(x: number): number;
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}

Different type parameters cannot be merged:

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function f<T extends number>(x: T): void;
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function f<T extends string>(x: T): void;
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function f(x: unknown): void {}