index.js

/** @type {import('eslint').Rule.RuleModule} */
export default {
	meta: {
		docs: {
			description:
				'Prefer early returns over full-body conditional wrapping in function declarations.',
			category: 'Best Practices',
			recommended: false,
			uri: 'https://github.com/cloudfour/eslint-config/blob/main/src/rules/prefer-early-return/README.md',
		},
		schema: [
			{
				type: 'object',
				properties: {
					maximumStatements: {
						type: 'integer',
					},
				},
				additionalProperties: false,
			},
		],
	},

	create(context) {
		const options = context.options[0] || { maximumStatements: 2 };
		const maxStatements = options.maximumStatements;

		/** @param {import('estree').Statement} consequent */
		function isOffendingConsequent(consequent) {
			return (
				(consequent.type === 'ExpressionStatement' && maxStatements === 0) ||
				(consequent.type === 'BlockStatement' &&
					consequent.body.length > maxStatements)
			);
		}

		/** @param {import('estree').Statement} statement */
		function isOffendingIfStatement(statement) {
			return (
				isLonelyIfStatement(statement) &&
				isOffendingConsequent(statement.consequent)
			);
		}

		/** @param {import('estree').BlockStatement} functionBody */
		function hasSimplifiableConditionalBody(functionBody) {
			const body = functionBody.body;
			return (
				functionBody.type === 'BlockStatement' &&
				body.length === 1 &&
				isOffendingIfStatement(body[0])
			);
		}

		/** @param {import('estree').FunctionDeclaration} functionNode */
		function checkFunctionBody(functionNode) {
			const body = functionNode.body;

			if (hasSimplifiableConditionalBody(body)) {
				context.report(
					body,
					'Prefer an early return to a conditionally-wrapped function body',
				);
			}
		}

		return {
			FunctionDeclaration: checkFunctionBody,
			FunctionExpression: checkFunctionBody,
			ArrowFunctionExpression: checkFunctionBody,
		};
	},
};

/**
 * @param {import('estree').Statement} statement
 * @returns {statement is import('estree').IfStatement}
 */
function isLonelyIfStatement(statement) {
	return statement.type === 'IfStatement' && statement.alternate === null;
}