May 2, 2025 · Mar 28, 2025 · Mar 31, 2025 · Apr 1, 2025 · Apr 1, 2025 · Apr 14, 2025
diff --git a/packages/eslint-plugin/src/rules/unified-signatures.ts b/packages/eslint-plugin/src/rules/unified-signatures.ts
      types1: readonly TSESTree.Parameter[],
      types2: readonly TSESTree.Parameter[],
    ): Unify | undefined {
      const firstParam1 = types1[0];
      const firstParam2 = types2[0];

      // exempt signatures with `this: void` from the rule
      if (isThisVoidParam(firstParam1) || isThisVoidParam(firstParam2)) {
        return undefined;
      }

      const index = getIndexOfFirstDifference(
        types1,
        types2,
        : undefined;
    }

    function isThisParam(param: TSESTree.Parameter | undefined): boolean {
      return (
        param != null &&
        param.type === AST_NODE_TYPES.Identifier &&
        param.name === 'this'
      );
    }

    function isThisVoidParam(param: TSESTree.Parameter | undefined) {
      return (
        isThisParam(param) &&
        (param as TSESTree.Identifier).typeAnnotation?.typeAnnotation.type ===
          AST_NODE_TYPES.TSVoidKeyword
      );
    }

    /**
     * Detect `a(): void` and `a(x: number): void`.
     * Returns the parameter declaration (`x: number` in this example) that should be optional/rest, and overload it's a part of.
      const shorter = sig1.length < sig2.length ? sig1 : sig2;
      const shorterSig = sig1.length < sig2.length ? a : b;

      const firstParam1 = sig1.at(0);
      const firstParam2 = sig2.at(0);
      // If one signature has explicit this type and another doesn't, they can't
      // be unified.
      if (isThisParam(firstParam1) !== isThisParam(firstParam2)) {
        return undefined;
      }

      // exempt signatures with `this: void` from the rule
      if (isThisVoidParam(firstParam1) || isThisVoidParam(firstParam2)) {
        return undefined;
      }

      // If one is has 2+ parameters more than the other, they must all be optional/rest.
      // Differ by optional parameters: f() and f(x), f() and f(x, ?y, ...z)
      // Not allowed: f() and f(x, y)
diff --git a/packages/eslint-plugin/tests/rules/unified-signatures.test.ts b/packages/eslint-plugin/tests/rules/unified-signatures.test.ts
      `,
      options: [{ ignoreOverloadsWithDifferentJSDoc: true }],
    },
    `
 function f(): void;
 function f(this: {}): void;
 function f(this: void | {}): void {}
    `,
    `
 function f(a: boolean): void;
 function f(this: {}, a: boolean): void;
 function f(this: void | {}, a: boolean): void {}
    `,
    `
 function f(this: void, a: boolean): void;
 function f(this: {}, a: boolean): void;
 function f(this: void | {}, a: boolean): void {}
    `,
  ],
  invalid: [
    {
      ],
      options: [{ ignoreOverloadsWithDifferentJSDoc: true }],
    },
    {
      code: `
 function f(this: {}, a: boolean): void;
 function f(this: {}, a: string): void;
 function f(this: {}, a: boolean | string): void {}
      `,
      errors: [
        {
          column: 22,
          line: 3,
          messageId: 'singleParameterDifference',
        },
      ],
    },
    {
      code: `
 function f(this: {}): void;
 function f(this: {}, a: string): void;
 function f(this: {}, a?: string): void {}
      `,
      errors: [
        {
          column: 22,
          line: 3,
          messageId: 'omittingSingleParameter',
        },
      ],
    },
    {
      code: `
 function f(this: string): void;
 function f(this: number): void;
 function f(this: string | number): void {}
      `,
      errors: [
        {
          column: 12,
          data: {
            failureStringStart:
              'These overloads can be combined into one signature',
            type1: 'string',
            type2: 'number',
          },
          line: 3,
          messageId: 'singleParameterDifference',
        },
      ],
    },
    {
      code: `
 function f(this: string, a: boolean): void;
 function f(this: number, a: boolean): void;
 function f(this: string | number, a: boolean): void {}
      `,
      errors: [
        {
          column: 12,
          data: {
            failureStringStart:
              'These overloads can be combined into one signature',
            type1: 'string',
            type2: 'number',
          },
          line: 3,
          messageId: 'singleParameterDifference',
        },
      ],
    },
  ],
 });
Original file line number	Diff line number	Diff line change
Expand Up		@@ -264,6 +264,14 @@ export default createRule<Options, MessageIds>({
		types1: readonly TSESTree.Parameter[],
		types2: readonly TSESTree.Parameter[],
		): Unify \| undefined {
		const firstParam1 = types1[0];
		const firstParam2 = types2[0];

		// exempt signatures with `this: void` from the rule
		if (isThisVoidParam(firstParam1) \|\| isThisVoidParam(firstParam2)) {
		return undefined;
		}

		const index = getIndexOfFirstDifference(
		types1,
		types2,
Expand DownExpand Up		@@ -294,6 +302,22 @@ export default createRule<Options, MessageIds>({
		: undefined;
		}

		function isThisParam(param: TSESTree.Parameter \| undefined): boolean {
Copy link Member kirkwaiblingerApr 28, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. FYI/PSA - I changed this not to be a type predicate since it's only valid in the`true` branch, not the`false` branch (the function returning`false` does not imply that`param` isnot a`TSESTree.Identifier`). Copy link ContributorAuthor jedwards1211May 2, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. Oh I didn't think about TS doing a negative refinement when the function returns`false`. It would be useful to be able to declare an assertion that only refines in the`true` case kirkwaiblinger reacted with thumbs up emoji
		return (
		param != null &&
		param.type === AST_NODE_TYPES.Identifier &&
		param.name === 'this'
		);
		}

		function isThisVoidParam(param: TSESTree.Parameter \| undefined) {
		return (
		isThisParam(param) &&
		(param as TSESTree.Identifier).typeAnnotation?.typeAnnotation.type ===
		AST_NODE_TYPES.TSVoidKeyword
		);
		}

		/**
		* Detect `a(): void` and `a(x: number): void`.
		* Returns the parameter declaration (`x: number` in this example) that should be optional/rest, and overload it's a part of.
Expand All		@@ -310,6 +334,19 @@ export default createRule<Options, MessageIds>({
		const shorter = sig1.length < sig2.length ? sig1 : sig2;
		const shorterSig = sig1.length < sig2.length ? a : b;

		const firstParam1 = sig1.at(0);
		const firstParam2 = sig2.at(0);
		// If one signature has explicit this type and another doesn't, they can't
		// be unified.
		if (isThisParam(firstParam1) !== isThisParam(firstParam2)) {
		return undefined;
		}

		// exempt signatures with `this: void` from the rule
		if (isThisVoidParam(firstParam1) \|\| isThisVoidParam(firstParam2)) {
		return undefined;
		}

		// If one is has 2+ parameters more than the other, they must all be optional/rest.
		// Differ by optional parameters: f() and f(x), f() and f(x, ?y, ...z)
		// Not allowed: f() and f(x, y)
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -380,6 +380,21 @@ declare function f(x: boolean): unknown;
		`,
		options: [{ ignoreOverloadsWithDifferentJSDoc: true }],
		},
		`
		function f(): void;
		function f(this: {}): void;
		function f(this: void \| {}): void {}
		`,
		`
		function f(a: boolean): void;
		function f(this: {}, a: boolean): void;
		function f(this: void \| {}, a: boolean): void {}
		`,
		`
		function f(this: void, a: boolean): void;
		function f(this: {}, a: boolean): void;
		function f(this: void \| {}, a: boolean): void {}
		`,
		],
		invalid: [
		{
Expand DownExpand Up		@@ -1136,5 +1151,73 @@ declare function f(x: boolean): unknown;
		],
		options: [{ ignoreOverloadsWithDifferentJSDoc: true }],
		},
		{
		code: `
		function f(this: {}, a: boolean): void;
		function f(this: {}, a: string): void;
		function f(this: {}, a: boolean \| string): void {}
		`,
		errors: [
		{
		column: 22,
		line: 3,
		messageId: 'singleParameterDifference',
		},
		],
		},
		{
		code: `
		function f(this: {}): void;
		function f(this: {}, a: string): void;
		function f(this: {}, a?: string): void {}
		`,
		errors: [
		{
		column: 22,
		line: 3,
		messageId: 'omittingSingleParameter',
		},
		],
		},
		{
		code: `
		function f(this: string): void;
		function f(this: number): void;
		function f(this: string \| number): void {}
		`,
		errors: [
		{
		column: 12,
		data: {
		failureStringStart:
		'These overloads can be combined into one signature',
		type1: 'string',
		type2: 'number',
		},
		line: 3,
		messageId: 'singleParameterDifference',
		},
		],
		},
		{
		code: `
		function f(this: string, a: boolean): void;
		function f(this: number, a: boolean): void;
		function f(this: string \| number, a: boolean): void {}
		`,
		errors: [
		{
		column: 12,
		data: {
		failureStringStart:
		'These overloads can be combined into one signature',
		type1: 'string',
		type2: 'number',
		},
		line: 3,
		messageId: 'singleParameterDifference',
		},
		],
		},
		],
		});