feat(require-event-prefix): implemented the rule

Author: marekdedic

Diff for: .changeset/rich-dogs-design.md

@@ -0,0 +1,5 @@
+---
+'eslint-plugin-svelte': minor
+---
+
+feat: added the `require-event-prefix` rule

Diff for: README.md

@@ -335,6 +335,7 @@ These rules relate to style guidelines, and are therefore quite subjective:
 | [svelte/no-spaces-around-equal-signs-in-attribute](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-spaces-around-equal-signs-in-attribute/) | disallow spaces around equal signs in attribute | :wrench: |
 | [svelte/prefer-class-directive](https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-class-directive/) | require class directives instead of ternary expressions | :wrench: |
 | [svelte/prefer-style-directive](https://sveltejs.github.io/eslint-plugin-svelte/rules/prefer-style-directive/) | require style directives instead of style attribute | :wrench: |
+| [svelte/require-event-prefix](https://sveltejs.github.io/eslint-plugin-svelte/rules/require-event-prefix/) | require component event names to start with "on" |  |
 | [svelte/shorthand-attribute](https://sveltejs.github.io/eslint-plugin-svelte/rules/shorthand-attribute/) | enforce use of shorthand syntax in attribute | :wrench: |
 | [svelte/shorthand-directive](https://sveltejs.github.io/eslint-plugin-svelte/rules/shorthand-directive/) | enforce use of shorthand syntax in directives | :wrench: |
 | [svelte/sort-attributes](https://sveltejs.github.io/eslint-plugin-svelte/rules/sort-attributes/) | enforce order of attributes | :wrench: |

Diff for: docs/rules.md

@@ -92,6 +92,7 @@ These rules relate to style guidelines, and are therefore quite subjective:
 | [svelte/no-spaces-around-equal-signs-in-attribute](./rules/no-spaces-around-equal-signs-in-attribute.md) | disallow spaces around equal signs in attribute                                    | :wrench: |
 | [svelte/prefer-class-directive](./rules/prefer-class-directive.md)                                       | require class directives instead of ternary expressions                            | :wrench: |
 | [svelte/prefer-style-directive](./rules/prefer-style-directive.md)                                       | require style directives instead of style attribute                                | :wrench: |
+| [svelte/require-event-prefix](./rules/require-event-prefix.md)                                           | require component event names to start with "on"                                   |          |
 | [svelte/shorthand-attribute](./rules/shorthand-attribute.md)                                             | enforce use of shorthand syntax in attribute                                       | :wrench: |
 | [svelte/shorthand-directive](./rules/shorthand-directive.md)                                             | enforce use of shorthand syntax in directives                                      | :wrench: |
 | [svelte/sort-attributes](./rules/sort-attributes.md)                                                     | enforce order of attributes                                                        | :wrench: |

Diff for: packages/eslint-plugin-svelte/src/rule-types.ts

@@ -306,6 +306,11 @@ export interface RuleOptions {
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/require-event-dispatcher-types/
    */
   'svelte/require-event-dispatcher-types'?: Linter.RuleEntry<[]>
+  /**
+   * require component event names to start with "on"
+   * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/require-event-prefix/
+   */
+  'svelte/require-event-prefix'?: Linter.RuleEntry<SvelteRequireEventPrefix>
   /**
    * require style attributes that can be optimized
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/require-optimized-style-attribute/
@@ -532,6 +537,10 @@ type SveltePreferConst = []|[{
   ignoreReadBeforeAssign?: boolean
   excludedRunes?: string[]
 }]
+// ----- svelte/require-event-prefix -----
+type SvelteRequireEventPrefix = []|[{
+  checkAsyncFunctions?: boolean
+}]
 // ----- svelte/shorthand-attribute -----
 type SvelteShorthandAttribute = []|[{
   prefer?: ("always" | "never")

Diff for: packages/eslint-plugin-svelte/src/rules/require-event-prefix.ts

@@ -0,0 +1,122 @@
+import { createRule } from '../utils/index.js';
+import { type TSTools, getTypeScriptTools } from '../utils/ts-utils/index.js';
+import {
+	type MethodSignature,
+	type Symbol,
+	SymbolFlags,
+	SyntaxKind,
+	type Type,
+	type TypeReferenceNode,
+	type PropertySignature
+} from 'typescript';
+import type { CallExpression } from 'estree';
+
+export default createRule('require-event-prefix', {
+	meta: {
+		docs: {
+			description: 'require component event names to start with "on"',
+			category: 'Stylistic Issues',
+			conflictWithPrettier: false,
+			recommended: false
+		},
+		schema: [
+			{
+				type: 'object',
+				properties: {
+					checkAsyncFunctions: {
+						type: 'boolean'
+					}
+				},
+				additionalProperties: false
+			}
+		],
+		messages: {
+			nonPrefixedFunction: 'Component event name must start with "on".'
+		},
+		type: 'suggestion',
+		conditions: [
+			{
+				svelteVersions: ['5'],
+				svelteFileTypes: ['.svelte']
+			}
+		]
+	},
+	create(context) {
+		const tsTools = getTypeScriptTools(context);
+		if (!tsTools) {
+			return {};
+		}
+
+		const checkAsyncFunctions = context.options[0]?.checkAsyncFunctions ?? false;
+
+		return {
+			CallExpression(node) {
+				const propsType = getPropsType(node, tsTools);
+				if (propsType === undefined) {
+					return;
+				}
+				for (const property of propsType.getProperties()) {
+					if (
+						isFunctionLike(property) &&
+						!property.getName().startsWith('on') &&
+						(checkAsyncFunctions || !isFunctionAsync(property))
+					) {
+						const declarationTsNode = property.getDeclarations()?.[0];
+						const declarationEstreeNode =
+							declarationTsNode !== undefined
+								? tsTools.service.tsNodeToESTreeNodeMap.get(declarationTsNode)
+								: undefined;
+						context.report({
+							node: declarationEstreeNode ?? node,
+							messageId: 'nonPrefixedFunction'
+						});
+					}
+				}
+			}
+		};
+	}
+});
+
+function getPropsType(node: CallExpression, tsTools: TSTools): Type | undefined {
+	if (
+		node.callee.type !== 'Identifier' ||
+		node.callee.name !== '$props' ||
+		node.parent.type !== 'VariableDeclarator'
+	) {
+		return undefined;
+	}
+
+	const tsNode = tsTools.service.esTreeNodeToTSNodeMap.get(node.parent.id);
+	if (tsNode === undefined) {
+		return undefined;
+	}
+
+	return tsTools.service.program.getTypeChecker().getTypeAtLocation(tsNode);
+}
+
+function isFunctionLike(functionSymbol: Symbol): boolean {
+	return (
+		(functionSymbol.getFlags() & SymbolFlags.Method) !== 0 ||
+		(functionSymbol.valueDeclaration?.kind === SyntaxKind.PropertySignature &&
+			(functionSymbol.valueDeclaration as PropertySignature).type?.kind === SyntaxKind.FunctionType)
+	);
+}
+
+function isFunctionAsync(functionSymbol: Symbol): boolean {
+	return (
+		functionSymbol.getDeclarations()?.some((declaration) => {
+			if (declaration.kind !== SyntaxKind.MethodSignature) {
+				return false;
+			}
+			const declarationType = (declaration as MethodSignature).type;
+			if (declarationType?.kind !== SyntaxKind.TypeReference) {
+				return false;
+			}
+			const declarationTypeName = (declarationType as TypeReferenceNode).typeName;
+			return (
+				declarationTypeName.kind === SyntaxKind.Identifier &&
+				declarationTypeName.escapedText === 'Promise'
+			);
+		}) ?? false
+	);
+}

Diff for: packages/eslint-plugin-svelte/src/utils/rules.ts

@@ -60,6 +60,7 @@ import preferDestructuredStoreProps from '../rules/prefer-destructured-store-pro
 import preferStyleDirective from '../rules/prefer-style-directive.js';
 import requireEachKey from '../rules/require-each-key.js';
 import requireEventDispatcherTypes from '../rules/require-event-dispatcher-types.js';
+import requireEventPrefix from '../rules/require-event-prefix.js';
 import requireOptimizedStyleAttribute from '../rules/require-optimized-style-attribute.js';
 import requireStoreCallbacksUseSetParam from '../rules/require-store-callbacks-use-set-param.js';
 import requireStoreReactiveAccess from '../rules/require-store-reactive-access.js';
@@ -133,6 +134,7 @@ export const rules = [
 	preferStyleDirective,
 	requireEachKey,
 	requireEventDispatcherTypes,
+	requireEventPrefix,
 	requireOptimizedStyleAttribute,
 	requireStoreCallbacksUseSetParam,
 	requireStoreReactiveAccess,