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node_modules/eslint/lib/rules/no-fallthrough.js

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+/**
+ * @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/iu;
+
+/**
+ * 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: {
+        type: "problem",
+
+        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",
+                        default: ""
+                    }
+                },
+                additionalProperties: false
+            }
+        ],
+        messages: {
+            case: "Expected a 'break' statement before 'case'.",
+            default: "Expected a 'break' statement before 'default'."
+        }
+    },
+
+    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, "u");
+        } 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({
+                        messageId: 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;
+                }
+            }
+        };
+    }
+};