no-else-return.js

/**
 * @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");

//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------

/**
 * @param {Token} token - The token to check
 * @returns {boolean} `true` if keyword let or const exist
 */
function isVariableDeclaration(token) {
    const variableDeclaration = token.value === "let" || token.value === "const";

    return token.type === "Keyword" && variableDeclaration;
}

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = {
    meta: {
        type: "suggestion",

        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);

                    /**
                     * If else block includes block scope local variable declaration [let or const],
                     * then it is not safe to remove else keyword [issue 11069]
                     */
                    const isDeclarationInside = sourceCode.getTokensBetween(startToken, endToken).findIndex(isVariableDeclaration) > -1;

                    if (isDeclarationInside) {
                        return null;
                    }

                    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

        };

    }
};