1 | /**
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2 | * @fileoverview A rule to suggest using of const declaration for variables that are never reassigned after declared.
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3 | * @author Toru Nagashima
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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 | //------------------------------------------------------------------------------
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9 | // Helpers
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10 | //------------------------------------------------------------------------------
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11 |
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12 | const PATTERN_TYPE = /^(?:.+?Pattern|RestElement|SpreadProperty|ExperimentalRestProperty|Property)$/;
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13 | const DECLARATION_HOST_TYPE = /^(?:Program|BlockStatement|SwitchCase)$/;
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14 | const DESTRUCTURING_HOST_TYPE = /^(?:VariableDeclarator|AssignmentExpression)$/;
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15 |
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16 | /**
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17 | * Adds multiple items to the tail of an array.
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18 | *
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19 | * @param {any[]} array - A destination to add.
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20 | * @param {any[]} values - Items to be added.
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21 | * @returns {void}
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22 | */
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23 | const pushAll = Function.apply.bind(Array.prototype.push);
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24 |
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25 | /**
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26 | * Checks whether a given node is located at `ForStatement.init` or not.
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27 | *
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28 | * @param {ASTNode} node - A node to check.
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29 | * @returns {boolean} `true` if the node is located at `ForStatement.init`.
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30 | */
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31 | function isInitOfForStatement(node) {
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32 | return node.parent.type === "ForStatement" && node.parent.init === node;
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33 | }
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34 |
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35 | /**
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36 | * Checks whether a given Identifier node becomes a VariableDeclaration or not.
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37 | *
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38 | * @param {ASTNode} identifier - An Identifier node to check.
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39 | * @returns {boolean} `true` if the node can become a VariableDeclaration.
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40 | */
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41 | function canBecomeVariableDeclaration(identifier) {
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42 | let node = identifier.parent;
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43 |
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44 | while (PATTERN_TYPE.test(node.type)) {
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45 | node = node.parent;
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46 | }
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47 |
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48 | return (
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49 | node.type === "VariableDeclarator" ||
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50 | (
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51 | node.type === "AssignmentExpression" &&
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52 | node.parent.type === "ExpressionStatement" &&
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53 | DECLARATION_HOST_TYPE.test(node.parent.parent.type)
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54 | )
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55 | );
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56 | }
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57 |
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58 | /**
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59 | * Gets an identifier node of a given variable.
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60 | *
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61 | * If the initialization exists or one or more reading references exist before
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62 | * the first assignment, the identifier node is the node of the declaration.
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63 | * Otherwise, the identifier node is the node of the first assignment.
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64 | *
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65 | * If the variable should not change to const, this function returns null.
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66 | * - If the variable is reassigned.
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67 | * - If the variable is never initialized nor assigned.
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68 | * - If the variable is initialized in a different scope from the declaration.
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69 | * - If the unique assignment of the variable cannot change to a declaration.
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70 | * e.g. `if (a) b = 1` / `return (b = 1)`
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71 | * - If the variable is declared in the global scope and `eslintUsed` is `true`.
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72 | * `/*exported foo` directive comment makes such variables. This rule does not
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73 | * warn such variables because this rule cannot distinguish whether the
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74 | * exported variables are reassigned or not.
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75 | *
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76 | * @param {eslint-scope.Variable} variable - A variable to get.
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77 | * @param {boolean} ignoreReadBeforeAssign -
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78 | * The value of `ignoreReadBeforeAssign` option.
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79 | * @returns {ASTNode|null}
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80 | * An Identifier node if the variable should change to const.
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81 | * Otherwise, null.
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82 | */
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83 | function getIdentifierIfShouldBeConst(variable, ignoreReadBeforeAssign) {
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84 | if (variable.eslintUsed && variable.scope.type === "global") {
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85 | return null;
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86 | }
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87 |
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88 | // Finds the unique WriteReference.
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89 | let writer = null;
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90 | let isReadBeforeInit = false;
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91 | const references = variable.references;
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92 |
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93 | for (let i = 0; i < references.length; ++i) {
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94 | const reference = references[i];
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95 |
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96 | if (reference.isWrite()) {
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97 | const isReassigned = (
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98 | writer !== null &&
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99 | writer.identifier !== reference.identifier
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100 | );
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101 |
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102 | if (isReassigned) {
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103 | return null;
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104 | }
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105 | writer = reference;
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106 |
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107 | } else if (reference.isRead() && writer === null) {
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108 | if (ignoreReadBeforeAssign) {
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109 | return null;
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110 | }
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111 | isReadBeforeInit = true;
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112 | }
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113 | }
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114 |
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115 | /*
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116 | * If the assignment is from a different scope, ignore it.
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117 | * If the assignment cannot change to a declaration, ignore it.
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118 | */
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119 | const shouldBeConst = (
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120 | writer !== null &&
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121 | writer.from === variable.scope &&
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122 | canBecomeVariableDeclaration(writer.identifier)
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123 | );
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124 |
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125 | if (!shouldBeConst) {
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126 | return null;
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127 | }
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128 | if (isReadBeforeInit) {
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129 | return variable.defs[0].name;
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130 | }
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131 | return writer.identifier;
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132 | }
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133 |
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134 | /**
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135 | * Gets the VariableDeclarator/AssignmentExpression node that a given reference
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136 | * belongs to.
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137 | * This is used to detect a mix of reassigned and never reassigned in a
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138 | * destructuring.
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139 | *
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140 | * @param {eslint-scope.Reference} reference - A reference to get.
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141 | * @returns {ASTNode|null} A VariableDeclarator/AssignmentExpression node or
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142 | * null.
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143 | */
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144 | function getDestructuringHost(reference) {
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145 | if (!reference.isWrite()) {
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146 | return null;
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147 | }
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148 | let node = reference.identifier.parent;
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149 |
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150 | while (PATTERN_TYPE.test(node.type)) {
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151 | node = node.parent;
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152 | }
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153 |
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154 | if (!DESTRUCTURING_HOST_TYPE.test(node.type)) {
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155 | return null;
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156 | }
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157 | return node;
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158 | }
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159 |
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160 | /**
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161 | * Groups by the VariableDeclarator/AssignmentExpression node that each
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162 | * reference of given variables belongs to.
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163 | * This is used to detect a mix of reassigned and never reassigned in a
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164 | * destructuring.
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165 | *
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166 | * @param {eslint-scope.Variable[]} variables - Variables to group by destructuring.
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167 | * @param {boolean} ignoreReadBeforeAssign -
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168 | * The value of `ignoreReadBeforeAssign` option.
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169 | * @returns {Map<ASTNode, ASTNode[]>} Grouped identifier nodes.
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170 | */
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171 | function groupByDestructuring(variables, ignoreReadBeforeAssign) {
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172 | const identifierMap = new Map();
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173 |
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174 | for (let i = 0; i < variables.length; ++i) {
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175 | const variable = variables[i];
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176 | const references = variable.references;
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177 | const identifier = getIdentifierIfShouldBeConst(variable, ignoreReadBeforeAssign);
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178 | let prevId = null;
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179 |
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180 | for (let j = 0; j < references.length; ++j) {
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181 | const reference = references[j];
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182 | const id = reference.identifier;
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183 |
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184 | /*
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185 | * Avoid counting a reference twice or more for default values of
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186 | * destructuring.
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187 | */
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188 | if (id === prevId) {
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189 | continue;
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190 | }
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191 | prevId = id;
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192 |
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193 | // Add the identifier node into the destructuring group.
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194 | const group = getDestructuringHost(reference);
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195 |
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196 | if (group) {
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197 | if (identifierMap.has(group)) {
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198 | identifierMap.get(group).push(identifier);
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199 | } else {
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200 | identifierMap.set(group, [identifier]);
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201 | }
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202 | }
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203 | }
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204 | }
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205 |
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206 | return identifierMap;
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207 | }
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208 |
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209 | /**
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210 | * Finds the nearest parent of node with a given type.
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211 | *
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212 | * @param {ASTNode} node – The node to search from.
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213 | * @param {string} type – The type field of the parent node.
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214 | * @param {Function} shouldStop – a predicate that returns true if the traversal should stop, and false otherwise.
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215 | * @returns {ASTNode} The closest ancestor with the specified type; null if no such ancestor exists.
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216 | */
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217 | function findUp(node, type, shouldStop) {
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218 | if (!node || shouldStop(node)) {
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219 | return null;
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220 | }
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221 | if (node.type === type) {
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222 | return node;
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223 | }
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224 | return findUp(node.parent, type, shouldStop);
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225 | }
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226 |
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227 | //------------------------------------------------------------------------------
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228 | // Rule Definition
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229 | //------------------------------------------------------------------------------
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230 |
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231 | module.exports = {
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232 | meta: {
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233 | docs: {
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234 | description: "require `const` declarations for variables that are never reassigned after declared",
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235 | category: "ECMAScript 6",
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236 | recommended: false,
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237 | url: "https://eslint.org/docs/rules/prefer-const"
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238 | },
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239 |
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240 | fixable: "code",
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241 |
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242 | schema: [
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243 | {
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244 | type: "object",
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245 | properties: {
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246 | destructuring: { enum: ["any", "all"] },
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247 | ignoreReadBeforeAssign: { type: "boolean" }
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248 | },
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249 | additionalProperties: false
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250 | }
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251 | ]
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252 | },
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253 |
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254 | create(context) {
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255 | const options = context.options[0] || {};
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256 | const sourceCode = context.getSourceCode();
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257 | const checkingMixedDestructuring = options.destructuring !== "all";
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258 | const ignoreReadBeforeAssign = options.ignoreReadBeforeAssign === true;
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259 | const variables = [];
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260 |
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261 | /**
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262 | * Reports given identifier nodes if all of the nodes should be declared
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263 | * as const.
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264 | *
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265 | * The argument 'nodes' is an array of Identifier nodes.
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266 | * This node is the result of 'getIdentifierIfShouldBeConst()', so it's
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267 | * nullable. In simple declaration or assignment cases, the length of
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268 | * the array is 1. In destructuring cases, the length of the array can
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269 | * be 2 or more.
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270 | *
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271 | * @param {(eslint-scope.Reference|null)[]} nodes -
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272 | * References which are grouped by destructuring to report.
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273 | * @returns {void}
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274 | */
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275 | function checkGroup(nodes) {
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276 | const nodesToReport = nodes.filter(Boolean);
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277 |
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278 | if (nodes.length && (checkingMixedDestructuring || nodesToReport.length === nodes.length)) {
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279 | const varDeclParent = findUp(nodes[0], "VariableDeclaration", parentNode => parentNode.type.endsWith("Statement"));
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280 | const shouldFix = varDeclParent &&
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281 |
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282 | /*
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283 | * If there are multiple variable declarations, like {let a = 1, b = 2}, then
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284 | * do not attempt to fix if one of the declarations should be `const`. It's
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285 | * too hard to know how the developer would want to automatically resolve the issue.
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286 | */
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287 | varDeclParent.declarations.length === 1 &&
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288 |
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289 | // Don't do a fix unless the variable is initialized (or it's in a for-in or for-of loop)
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290 | (varDeclParent.parent.type === "ForInStatement" || varDeclParent.parent.type === "ForOfStatement" || varDeclParent.declarations[0].init) &&
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291 |
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292 | /*
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293 | * If options.destucturing is "all", then this warning will not occur unless
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294 | * every assignment in the destructuring should be const. In that case, it's safe
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295 | * to apply the fix.
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296 | */
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297 | nodesToReport.length === nodes.length;
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298 |
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299 | nodesToReport.forEach(node => {
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300 | context.report({
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301 | node,
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302 | message: "'{{name}}' is never reassigned. Use 'const' instead.",
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303 | data: node,
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304 | fix: shouldFix ? fixer => fixer.replaceText(sourceCode.getFirstToken(varDeclParent), "const") : null
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305 | });
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306 | });
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307 | }
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308 | }
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309 |
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310 | return {
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311 | "Program:exit"() {
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312 | groupByDestructuring(variables, ignoreReadBeforeAssign).forEach(checkGroup);
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313 | },
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314 |
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315 | VariableDeclaration(node) {
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316 | if (node.kind === "let" && !isInitOfForStatement(node)) {
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317 | pushAll(variables, context.getDeclaredVariables(node));
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318 | }
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319 | }
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320 | };
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321 | }
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322 | };
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