/** * @fileoverview Rule to flag `else` after a `return` in `if` * @author Ian Christian Myers */ "use strict"; //------------------------------------------------------------------------------ // Requirements //------------------------------------------------------------------------------ const astUtils = require("../util/ast-utils"); const FixTracker = require("../util/fix-tracker"); //------------------------------------------------------------------------------ // Rule Definition //------------------------------------------------------------------------------ module.exports = { meta: { docs: { description: "disallow `else` blocks after `return` statements in `if` statements", category: "Best Practices", recommended: false, url: "https://eslint.org/docs/rules/no-else-return" }, schema: [{ type: "object", properties: { allowElseIf: { type: "boolean" } }, additionalProperties: false }], fixable: "code", messages: { unexpected: "Unnecessary 'else' after 'return'." } }, create(context) { //-------------------------------------------------------------------------- // Helpers //-------------------------------------------------------------------------- /** * Display the context report if rule is violated * * @param {Node} node The 'else' node * @returns {void} */ function displayReport(node) { context.report({ node, messageId: "unexpected", fix: fixer => { const sourceCode = context.getSourceCode(); const startToken = sourceCode.getFirstToken(node); const elseToken = sourceCode.getTokenBefore(startToken); const source = sourceCode.getText(node); const lastIfToken = sourceCode.getTokenBefore(elseToken); let fixedSource, firstTokenOfElseBlock; if (startToken.type === "Punctuator" && startToken.value === "{") { firstTokenOfElseBlock = sourceCode.getTokenAfter(startToken); } else { firstTokenOfElseBlock = startToken; } /* * If the if block does not have curly braces and does not end in a semicolon * and the else block starts with (, [, /, +, ` or -, then it is not * safe to remove the else keyword, because ASI will not add a semicolon * after the if block */ const ifBlockMaybeUnsafe = node.parent.consequent.type !== "BlockStatement" && lastIfToken.value !== ";"; const elseBlockUnsafe = /^[([/+`-]/.test(firstTokenOfElseBlock.value); if (ifBlockMaybeUnsafe && elseBlockUnsafe) { return null; } const endToken = sourceCode.getLastToken(node); const lastTokenOfElseBlock = sourceCode.getTokenBefore(endToken); if (lastTokenOfElseBlock.value !== ";") { const nextToken = sourceCode.getTokenAfter(endToken); const nextTokenUnsafe = nextToken && /^[([/+`-]/.test(nextToken.value); const nextTokenOnSameLine = nextToken && nextToken.loc.start.line === lastTokenOfElseBlock.loc.start.line; /* * If the else block contents does not end in a semicolon, * and the else block starts with (, [, /, +, ` or -, then it is not * safe to remove the else block, because ASI will not add a semicolon * after the remaining else block contents */ if (nextTokenUnsafe || (nextTokenOnSameLine && nextToken.value !== "}")) { return null; } } if (startToken.type === "Punctuator" && startToken.value === "{") { fixedSource = source.slice(1, -1); } else { fixedSource = source; } /* * Extend the replacement range to include the entire * function to avoid conflicting with no-useless-return. * https://github.com/eslint/eslint/issues/8026 */ return new FixTracker(fixer, sourceCode) .retainEnclosingFunction(node) .replaceTextRange([elseToken.range[0], node.range[1]], fixedSource); } }); } /** * Check to see if the node is a ReturnStatement * * @param {Node} node The node being evaluated * @returns {boolean} True if node is a return */ function checkForReturn(node) { return node.type === "ReturnStatement"; } /** * Naive return checking, does not iterate through the whole * BlockStatement because we make the assumption that the ReturnStatement * will be the last node in the body of the BlockStatement. * * @param {Node} node The consequent/alternate node * @returns {boolean} True if it has a return */ function naiveHasReturn(node) { if (node.type === "BlockStatement") { const body = node.body, lastChildNode = body[body.length - 1]; return lastChildNode && checkForReturn(lastChildNode); } return checkForReturn(node); } /** * Check to see if the node is valid for evaluation, * meaning it has an else. * * @param {Node} node The node being evaluated * @returns {boolean} True if the node is valid */ function hasElse(node) { return node.alternate && node.consequent; } /** * If the consequent is an IfStatement, check to see if it has an else * and both its consequent and alternate path return, meaning this is * a nested case of rule violation. If-Else not considered currently. * * @param {Node} node The consequent node * @returns {boolean} True if this is a nested rule violation */ function checkForIf(node) { return node.type === "IfStatement" && hasElse(node) && naiveHasReturn(node.alternate) && naiveHasReturn(node.consequent); } /** * Check the consequent/body node to make sure it is not * a ReturnStatement or an IfStatement that returns on both * code paths. * * @param {Node} node The consequent or body node * @param {Node} alternate The alternate node * @returns {boolean} `true` if it is a Return/If node that always returns. */ function checkForReturnOrIf(node) { return checkForReturn(node) || checkForIf(node); } /** * Check whether a node returns in every codepath. * @param {Node} node The node to be checked * @returns {boolean} `true` if it returns on every codepath. */ function alwaysReturns(node) { if (node.type === "BlockStatement") { // If we have a BlockStatement, check each consequent body node. return node.body.some(checkForReturnOrIf); } /* * If not a block statement, make sure the consequent isn't a * ReturnStatement or an IfStatement with returns on both paths. */ return checkForReturnOrIf(node); } /** * Check the if statement, but don't catch else-if blocks. * @returns {void} * @param {Node} node The node for the if statement to check * @private */ function checkIfWithoutElse(node) { const parent = node.parent; /* * Fixing this would require splitting one statement into two, so no error should * be reported if this node is in a position where only one statement is allowed. */ if (!astUtils.STATEMENT_LIST_PARENTS.has(parent.type)) { return; } const consequents = []; let alternate; for (let currentNode = node; currentNode.type === "IfStatement"; currentNode = currentNode.alternate) { if (!currentNode.alternate) { return; } consequents.push(currentNode.consequent); alternate = currentNode.alternate; } if (consequents.every(alwaysReturns)) { displayReport(alternate); } } /** * Check the if statement * @returns {void} * @param {Node} node The node for the if statement to check * @private */ function checkIfWithElse(node) { const parent = node.parent; /* * Fixing this would require splitting one statement into two, so no error should * be reported if this node is in a position where only one statement is allowed. */ if (!astUtils.STATEMENT_LIST_PARENTS.has(parent.type)) { return; } const alternate = node.alternate; if (alternate && alwaysReturns(node.consequent)) { displayReport(alternate); } } const allowElseIf = !(context.options[0] && context.options[0].allowElseIf === false); //-------------------------------------------------------------------------- // Public API //-------------------------------------------------------------------------- return { "IfStatement:exit": allowElseIf ? checkIfWithoutElse : checkIfWithElse }; } };