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/**
* @fileoverview Rule to flag fall-through cases in switch statements.
* @author Matt DuVall <http://mattduvall.com/>
*/
"use strict";
//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------
const { directivesPattern } = require("../shared/directives");
//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------
const DEFAULT_FALLTHROUGH_COMMENT = /falls?\s?through/iu;
/**
* Checks all segments in a set and returns true if any are reachable.
* @param {Set<CodePathSegment>} segments The segments to check.
* @returns {boolean} True if any segment is reachable; false otherwise.
*/
function isAnySegmentReachable(segments) {
for (const segment of segments) {
if (segment.reachable) {
return true;
}
}
return false;
}
/**
* Checks whether or not a given comment string is really a fallthrough comment and not an ESLint directive.
* @param {string} comment The comment string to check.
* @param {RegExp} fallthroughCommentPattern The regular expression used for checking for fallthrough comments.
* @returns {boolean} `true` if the comment string is truly a fallthrough comment.
*/
function isFallThroughComment(comment, fallthroughCommentPattern) {
return (
fallthroughCommentPattern.test(comment) &&
!directivesPattern.test(comment.trim())
);
}
/**
* Checks whether or not a given case has a fallthrough comment.
* @param {ASTNode} caseWhichFallsThrough SwitchCase node which falls through.
* @param {ASTNode} subsequentCase The case after caseWhichFallsThrough.
* @param {RuleContext} context A rule context which stores comments.
* @param {RegExp} fallthroughCommentPattern A pattern to match comment to.
* @returns {null | object} the comment if the case has a valid fallthrough comment, otherwise null
*/
function getFallthroughComment(
caseWhichFallsThrough,
subsequentCase,
context,
fallthroughCommentPattern,
) {
const sourceCode = context.sourceCode;
if (
caseWhichFallsThrough.consequent.length === 1 &&
caseWhichFallsThrough.consequent[0].type === "BlockStatement"
) {
const trailingCloseBrace = sourceCode.getLastToken(
caseWhichFallsThrough.consequent[0],
);
const commentInBlock = sourceCode
.getCommentsBefore(trailingCloseBrace)
.pop();
if (
commentInBlock &&
isFallThroughComment(
commentInBlock.value,
fallthroughCommentPattern,
)
) {
return commentInBlock;
}
}
const comment = sourceCode.getCommentsBefore(subsequentCase).pop();
if (
comment &&
isFallThroughComment(comment.value, fallthroughCommentPattern)
) {
return comment;
}
return null;
}
/**
* 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
//------------------------------------------------------------------------------
/** @type {import('../types').Rule.RuleModule} */
module.exports = {
meta: {
type: "problem",
defaultOptions: [
{
allowEmptyCase: false,
reportUnusedFallthroughComment: false,
},
],
docs: {
description: "Disallow fallthrough of `case` statements",
recommended: true,
url: "https://eslint.org/docs/latest/rules/no-fallthrough",
},
schema: [
{
type: "object",
properties: {
commentPattern: {
type: "string",
},
allowEmptyCase: {
type: "boolean",
},
reportUnusedFallthroughComment: {
type: "boolean",
},
},
additionalProperties: false,
},
],
messages: {
unusedFallthroughComment:
"Found a comment that would permit fallthrough, but case cannot fall through.",
case: "Expected a 'break' statement before 'case'.",
default: "Expected a 'break' statement before 'default'.",
},
},
create(context) {
const codePathSegments = [];
let currentCodePathSegments = new Set();
const sourceCode = context.sourceCode;
const [
{ allowEmptyCase, commentPattern, reportUnusedFallthroughComment },
] = context.options;
const fallthroughCommentPattern = commentPattern
? new RegExp(commentPattern, "u")
: DEFAULT_FALLTHROUGH_COMMENT;
/*
* We need to use leading comments of the next SwitchCase node because
* trailing comments is wrong if semicolons are omitted.
*/
let previousCase = null;
return {
onCodePathStart() {
codePathSegments.push(currentCodePathSegments);
currentCodePathSegments = new Set();
},
onCodePathEnd() {
currentCodePathSegments = codePathSegments.pop();
},
onUnreachableCodePathSegmentStart(segment) {
currentCodePathSegments.add(segment);
},
onUnreachableCodePathSegmentEnd(segment) {
currentCodePathSegments.delete(segment);
},
onCodePathSegmentStart(segment) {
currentCodePathSegments.add(segment);
},
onCodePathSegmentEnd(segment) {
currentCodePathSegments.delete(segment);
},
SwitchCase(node) {
/*
* Checks whether or not there is a fallthrough comment.
* And reports the previous fallthrough node if that does not exist.
*/
if (previousCase && previousCase.node.parent === node.parent) {
const previousCaseFallthroughComment =
getFallthroughComment(
previousCase.node,
node,
context,
fallthroughCommentPattern,
);
if (
previousCase.isFallthrough &&
!previousCaseFallthroughComment
) {
context.report({
messageId: node.test ? "case" : "default",
node,
});
} else if (
reportUnusedFallthroughComment &&
!previousCase.isSwitchExitReachable &&
previousCaseFallthroughComment
) {
context.report({
messageId: "unusedFallthroughComment",
node: previousCaseFallthroughComment,
});
}
}
previousCase = 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.
*/
const isSwitchExitReachable = isAnySegmentReachable(
currentCodePathSegments,
);
const isFallthrough =
isSwitchExitReachable &&
(node.consequent.length > 0 ||
(!allowEmptyCase &&
hasBlankLinesBetween(node, nextToken))) &&
node.parent.cases.at(-1) !== node;
previousCase = {
node,
isSwitchExitReachable,
isFallthrough,
};
},
};
},
};
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