1 | /**
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2 | * @fileoverview Rule to flag fall-through cases in switch statements.
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3 | * @author Matt DuVall <http://mattduvall.com/>
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4 | */
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5 | ;
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6 |
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7 | //------------------------------------------------------------------------------
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8 | // Requirements
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9 | //------------------------------------------------------------------------------
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10 |
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11 | const lodash = require("lodash");
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12 |
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13 | //------------------------------------------------------------------------------
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14 | // Helpers
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15 | //------------------------------------------------------------------------------
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16 |
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17 | const DEFAULT_FALLTHROUGH_COMMENT = /falls?\s?through/i;
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18 |
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19 | /**
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20 | * Checks whether or not a given node has a fallthrough comment.
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21 | * @param {ASTNode} node - A SwitchCase node to get comments.
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22 | * @param {RuleContext} context - A rule context which stores comments.
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23 | * @param {RegExp} fallthroughCommentPattern - A pattern to match comment to.
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24 | * @returns {boolean} `true` if the node has a valid fallthrough comment.
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25 | */
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26 | function hasFallthroughComment(node, context, fallthroughCommentPattern) {
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27 | const sourceCode = context.getSourceCode();
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28 | const comment = lodash.last(sourceCode.getCommentsBefore(node));
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29 |
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30 | return Boolean(comment && fallthroughCommentPattern.test(comment.value));
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31 | }
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32 |
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33 | /**
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34 | * Checks whether or not a given code path segment is reachable.
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35 | * @param {CodePathSegment} segment - A CodePathSegment to check.
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36 | * @returns {boolean} `true` if the segment is reachable.
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37 | */
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38 | function isReachable(segment) {
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39 | return segment.reachable;
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40 | }
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41 |
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42 | /**
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43 | * Checks whether a node and a token are separated by blank lines
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44 | * @param {ASTNode} node - The node to check
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45 | * @param {Token} token - The token to compare against
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46 | * @returns {boolean} `true` if there are blank lines between node and token
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47 | */
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48 | function hasBlankLinesBetween(node, token) {
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49 | return token.loc.start.line > node.loc.end.line + 1;
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50 | }
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51 |
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52 | //------------------------------------------------------------------------------
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53 | // Rule Definition
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54 | //------------------------------------------------------------------------------
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55 |
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56 | module.exports = {
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57 | meta: {
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58 | docs: {
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59 | description: "disallow fallthrough of `case` statements",
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60 | category: "Best Practices",
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61 | recommended: true,
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62 | url: "https://eslint.org/docs/rules/no-fallthrough"
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63 | },
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64 |
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65 | schema: [
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66 | {
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67 | type: "object",
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68 | properties: {
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69 | commentPattern: {
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70 | type: "string"
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71 | }
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72 | },
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73 | additionalProperties: false
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74 | }
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75 | ]
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76 | },
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77 |
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78 | create(context) {
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79 | const options = context.options[0] || {};
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80 | let currentCodePath = null;
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81 | const sourceCode = context.getSourceCode();
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82 |
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83 | /*
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84 | * We need to use leading comments of the next SwitchCase node because
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85 | * trailing comments is wrong if semicolons are omitted.
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86 | */
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87 | let fallthroughCase = null;
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88 | let fallthroughCommentPattern = null;
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89 |
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90 | if (options.commentPattern) {
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91 | fallthroughCommentPattern = new RegExp(options.commentPattern);
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92 | } else {
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93 | fallthroughCommentPattern = DEFAULT_FALLTHROUGH_COMMENT;
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94 | }
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95 |
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96 | return {
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97 | onCodePathStart(codePath) {
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98 | currentCodePath = codePath;
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99 | },
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100 | onCodePathEnd() {
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101 | currentCodePath = currentCodePath.upper;
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102 | },
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103 |
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104 | SwitchCase(node) {
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105 |
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106 | /*
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107 | * Checks whether or not there is a fallthrough comment.
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108 | * And reports the previous fallthrough node if that does not exist.
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109 | */
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110 | if (fallthroughCase && !hasFallthroughComment(node, context, fallthroughCommentPattern)) {
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111 | context.report({
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112 | message: "Expected a 'break' statement before '{{type}}'.",
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113 | data: { type: node.test ? "case" : "default" },
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114 | node
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115 | });
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116 | }
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117 | fallthroughCase = null;
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118 | },
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119 |
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120 | "SwitchCase:exit"(node) {
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121 | const nextToken = sourceCode.getTokenAfter(node);
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122 |
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123 | /*
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124 | * `reachable` meant fall through because statements preceded by
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125 | * `break`, `return`, or `throw` are unreachable.
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126 | * And allows empty cases and the last case.
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127 | */
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128 | if (currentCodePath.currentSegments.some(isReachable) &&
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129 | (node.consequent.length > 0 || hasBlankLinesBetween(node, nextToken)) &&
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130 | lodash.last(node.parent.cases) !== node) {
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131 | fallthroughCase = node;
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132 | }
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133 | }
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134 | };
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135 | }
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136 | };
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