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feat(eslint-plugin): add prefer-function-type rule (#222)
feat(eslint-plugin): add prefer-function-type rule
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Diff for: ‎packages/eslint-plugin/README.md

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@@ -144,6 +144,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
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| [`@typescript-eslint/no-use-before-define`](./docs/rules/no-use-before-define.md) | Disallow the use of variables before they are defined | :heavy_check_mark: | |
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| [`@typescript-eslint/no-useless-constructor`](./docs/rules/no-useless-constructor.md) | Disallow unnecessary constructors | | |
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| [`@typescript-eslint/no-var-requires`](./docs/rules/no-var-requires.md) | Disallows the use of require statements except in import statements (`no-var-requires` from TSLint) | :heavy_check_mark: | |
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| [`@typescript-eslint/prefer-function-type`](./docs/rules/prefer-function-type.md) | Use function types instead of interfaces with call signatures (`callable-types` from TSLint) | | :wrench: |
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| [`@typescript-eslint/prefer-interface`](./docs/rules/prefer-interface.md) | Prefer an interface declaration over a type literal (type T = { ... }) (`interface-over-type-literal` from TSLint) | :heavy_check_mark: | :wrench: |
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| [`@typescript-eslint/prefer-namespace-keyword`](./docs/rules/prefer-namespace-keyword.md) | Require the use of the `namespace` keyword instead of the `module` keyword to declare custom TypeScript modules. (`no-internal-module` from TSLint) | :heavy_check_mark: | :wrench: |
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| [`@typescript-eslint/promise-function-async`](./docs/rules/promise-function-async.md) | Requires any function or method that returns a Promise to be marked async. (`promise-function-async` from TSLint) | :heavy_check_mark: | |

Diff for: ‎packages/eslint-plugin/ROADMAP.md

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| [`arrow-parens`] | 🌟 | [`arrow-parens`][arrow-parens] |
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| [`arrow-return-shorthand`] | 🌟 | [`arrow-body-style`][arrow-body-style] |
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| [`binary-expression-operand-order`] | 🌟 | [`yoda`][yoda] |
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| [`callable-types`] | 🛑 | N/A |
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| [`callable-types`] | ✅ | [`@typescript-eslint/prefer-function-type`] |
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| [`class-name`] | ✅ | [`@typescript-eslint/class-name-casing`] |
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| [`comment-format`] | 🌟 | [`capitalized-comments`][capitalized-comments] & [`spaced-comment`][spaced-comment] |
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| [`completed-docs`] | 🔌 | [`eslint-plugin-jsdoc`][plugin:jsdoc] |
@@ -587,6 +587,7 @@ Relevant plugins: [`chai-expect-keywords`](https://github.com/gavinaiken/eslint-
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[`@typescript-eslint/member-delimiter-style`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/member-delimiter-style.md
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[`@typescript-eslint/prefer-interface`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-interface.md
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[`@typescript-eslint/no-array-constructor`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-array-constructor.md
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[`@typescript-eslint/prefer-function-type`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-function-type.md
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[`@typescript-eslint/no-for-in-array`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-for-in-array.md
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<!-- eslint-plugin-import -->

Diff for: ‎packages/eslint-plugin/docs/rules/prefer-function-type.md

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# Use function types instead of interfaces with call signatures (prefer-function-type)
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## Rule Details
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This rule suggests using a function type instead of an interface or object type literal with a single call signature.
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Examples of **incorrect** code for this rule:
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```ts
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interface Foo {
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(): string;
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}
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```
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```ts
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function foo(bar: { (): number }): number {
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return bar();
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}
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```
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```ts
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interface Foo extends Function {
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(): void;
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}
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```
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Examples of **correct** code for this rule:
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```ts
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interface Foo {
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(): void;
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bar: number;
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}
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```
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```ts
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function foo(bar: { (): string; baz: number }): string {
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return bar();
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}
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```
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```ts
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interface Foo {
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bar: string;
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}
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interface Bar extends Foo {
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(): void;
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}
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```
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## When Not To Use It
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If you specifically want to use an interface or type literal with a single call signature for stylistic reasons, you can disable this rule.
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## Further Reading
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- TSLint: [`callable-types`](https://palantir.github.io/tslint/rules/callable-types/)

Diff for: ‎packages/eslint-plugin/lib/rules/prefer-function-type.js

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/**
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* @fileoverview Use function types instead of interfaces with call signatures
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* @author Benjamin Lichtman
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*/
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'use strict';
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const util = require('../util');
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/**
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* @typedef {import("eslint").Rule.RuleModule} RuleModule
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* @typedef {import("estree").Node} ESTreeNode
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*/
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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/**
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* @type {RuleModule}
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*/
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module.exports = {
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meta: {
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docs: {
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description:
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'Use function types instead of interfaces with call signatures',
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category: 'TypeScript',
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recommended: false,
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extraDescription: [util.tslintRule('prefer-function-type')],
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url: util.metaDocsUrl('prefer-function-type')
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},
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fixable: 'code',
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messages: {
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functionTypeOverCallableType:
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"{{ type }} has only a call signature - use '{{ sigSuggestion }}' instead."
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},
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schema: [],
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type: 'suggestion'
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},
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create(context) {
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const sourceCode = context.getSourceCode();
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//----------------------------------------------------------------------
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// Helpers
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//----------------------------------------------------------------------
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/**
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* Checks if there is no supertype or if the supertype is 'Function'
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* @param {ESTreeNode} node The node being checked
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* @returns {boolean} Returns true iff there is no supertype or if the supertype is 'Function'
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*/
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function noSupertype(node) {
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if (!node.extends || node.extends.length === 0) {
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return true;
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}
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if (node.extends.length !== 1) {
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return false;
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}
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const expr = node.extends[0].expression;
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return expr.type === 'Identifier' && expr.name === 'Function';
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}
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/**
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* @param {ESTreeNode} parent The parent of the call signature causing the diagnostic
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* @returns {boolean} true iff the parent node needs to be wrapped for readability
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*/
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function shouldWrapSuggestion(parent) {
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switch (parent.type) {
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case 'TSUnionType':
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case 'TSIntersectionType':
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case 'TSArrayType':
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return true;
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default:
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return false;
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}
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}
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/**
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* @param {ESTreeNode} call The call signature causing the diagnostic
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* @param {ESTreeNode} parent The parent of the call
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* @returns {string} The suggestion to report
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*/
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function renderSuggestion(call, parent) {
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const start = call.range[0];
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const colonPos = call.returnType.range[0] - start;
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const text = sourceCode.getText().slice(start, call.range[1]);
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let suggestion = `${text.slice(0, colonPos)} =>${text.slice(
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colonPos + 1
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)}`;
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if (shouldWrapSuggestion(parent.parent)) {
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suggestion = `(${suggestion})`;
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}
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if (parent.type === 'TSInterfaceDeclaration') {
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if (typeof parent.typeParameters !== 'undefined') {
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return `type ${sourceCode
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.getText()
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.slice(
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parent.id.range[0],
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parent.typeParameters.range[1]
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)} = ${suggestion}`;
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}
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return `type ${parent.id.name} = ${suggestion}`;
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}
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return suggestion.endsWith(';') ? suggestion.slice(0, -1) : suggestion;
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}
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/**
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* @param {ESTreeNode} member The TypeElement being checked
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* @param {ESTreeNode} node The parent of member being checked
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* @returns {void}
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*/
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function checkMember(member, node) {
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if (
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(member.type === 'TSCallSignatureDeclaration' ||
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member.type === 'TSConstructSignatureDeclaration') &&
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typeof member.returnType !== 'undefined'
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) {
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const suggestion = renderSuggestion(member, node);
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const fixStart =
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node.type === 'TSTypeLiteral'
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? node.range[0]
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: sourceCode
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.getTokens(node)
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.filter(
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token =>
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token.type === 'Keyword' && token.value === 'interface'
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)[0].range[0];
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context.report({
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node: member,
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messageId: 'functionTypeOverCallableType',
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data: {
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type: node.type === 'TSTypeLiteral' ? 'Type literal' : 'Interface',
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sigSuggestion: suggestion
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},
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fix(fixer) {
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return fixer.replaceTextRange(
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[fixStart, node.range[1]],
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suggestion
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);
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}
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});
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}
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}
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//----------------------------------------------------------------------
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// Public
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//----------------------------------------------------------------------
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return {
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/**
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* @param {TSInterfaceDeclaration} node The node being checked
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* @returns {void}
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*/
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TSInterfaceDeclaration(node) {
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if (noSupertype(node) && node.body.body.length === 1) {
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checkMember(node.body.body[0], node);
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}
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},
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/**
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* @param {TSTypeLiteral} node The node being checked
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* @returns {void}
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*/
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'TSTypeLiteral[members.length = 1]'(node) {
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checkMember(node.members[0], node);
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}
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};
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}
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};

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