eslint/lib/rules/no-fallthrough.js

/**
 * @fileoverview Rule to flag fall-through cases in switch statements.
 * @author Matt DuVall <http://mattduvall.com/>
 */
"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const lodash = require("lodash");

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

const DEFAULT_FALLTHROUGH_COMMENT = /falls?\s?through/i;

/**
 * Checks whether or not a given node has a fallthrough comment.
 * @param {ASTNode} node - A SwitchCase node to get comments.
 * @param {RuleContext} context - A rule context which stores comments.
 * @param {RegExp} fallthroughCommentPattern - A pattern to match comment to.
 * @returns {boolean} `true` if the node has a valid fallthrough comment.
 */
function hasFallthroughComment(node, context, fallthroughCommentPattern) {
    const sourceCode = context.getSourceCode();
    const comment = lodash.last(sourceCode.getCommentsBefore(node));

    return Boolean(comment && fallthroughCommentPattern.test(comment.value));
}

/**
 * Checks whether or not a given code path segment is reachable.
 * @param {CodePathSegment} segment - A CodePathSegment to check.
 * @returns {boolean} `true` if the segment is reachable.
 */
function isReachable(segment) {
    return segment.reachable;
}

/**
 * Checks whether a node and a token are separated by blank lines
 * @param {ASTNode} node - The node to check
 * @param {Token} token - The token to compare against
 * @returns {boolean} `true` if there are blank lines between node and token
 */
function hasBlankLinesBetween(node, token) {
    return token.loc.start.line > node.loc.end.line + 1;
}

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

module.exports = {
    meta: {
        docs: {
            description: "disallow fallthrough of `case` statements",
            category: "Best Practices",
            recommended: true,
            url: "https://eslint.org/docs/rules/no-fallthrough"
        },

        schema: [
            {
                type: "object",
                properties: {
                    commentPattern: {
                        type: "string"
                    }
                },
                additionalProperties: false
            }
        ]
    },

    create(context) {
        const options = context.options[0] || {};
        let currentCodePath = null;
        const sourceCode = context.getSourceCode();

        /*
         * We need to use leading comments of the next SwitchCase node because
         * trailing comments is wrong if semicolons are omitted.
         */
        let fallthroughCase = null;
        let fallthroughCommentPattern = null;

        if (options.commentPattern) {
            fallthroughCommentPattern = new RegExp(options.commentPattern);
        } else {
            fallthroughCommentPattern = DEFAULT_FALLTHROUGH_COMMENT;
        }

        return {
            onCodePathStart(codePath) {
                currentCodePath = codePath;
            },
            onCodePathEnd() {
                currentCodePath = currentCodePath.upper;
            },

            SwitchCase(node) {

                /*
                 * Checks whether or not there is a fallthrough comment.
                 * And reports the previous fallthrough node if that does not exist.
                 */
                if (fallthroughCase && !hasFallthroughComment(node, context, fallthroughCommentPattern)) {
                    context.report({
                        message: "Expected a 'break' statement before '{{type}}'.",
                        data: { type: node.test ? "case" : "default" },
                        node
                    });
                }
                fallthroughCase = null;
            },

            "SwitchCase:exit"(node) {
                const nextToken = sourceCode.getTokenAfter(node);

                /*
                 * `reachable` meant fall through because statements preceded by
                 * `break`, `return`, or `throw` are unreachable.
                 * And allows empty cases and the last case.
                 */
                if (currentCodePath.currentSegments.some(isReachable) &&
                    (node.consequent.length > 0 || hasBlankLinesBetween(node, nextToken)) &&
                    lodash.last(node.parent.cases) !== node) {
                    fallthroughCase = node;
                }
            }
        };
    }
};