323 lines
11 KiB
JavaScript
323 lines
11 KiB
JavaScript
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/**
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* @fileoverview A rule to suggest using of const declaration for variables that are never reassigned after declared.
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* @author Toru Nagashima
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*/
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"use strict";
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//------------------------------------------------------------------------------
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// Helpers
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//------------------------------------------------------------------------------
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const PATTERN_TYPE = /^(?:.+?Pattern|RestElement|SpreadProperty|ExperimentalRestProperty|Property)$/;
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const DECLARATION_HOST_TYPE = /^(?:Program|BlockStatement|SwitchCase)$/;
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const DESTRUCTURING_HOST_TYPE = /^(?:VariableDeclarator|AssignmentExpression)$/;
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/**
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* Adds multiple items to the tail of an array.
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*
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* @param {any[]} array - A destination to add.
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* @param {any[]} values - Items to be added.
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* @returns {void}
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*/
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const pushAll = Function.apply.bind(Array.prototype.push);
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/**
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* Checks whether a given node is located at `ForStatement.init` or not.
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*
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* @param {ASTNode} node - A node to check.
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* @returns {boolean} `true` if the node is located at `ForStatement.init`.
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*/
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function isInitOfForStatement(node) {
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return node.parent.type === "ForStatement" && node.parent.init === node;
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}
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/**
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* Checks whether a given Identifier node becomes a VariableDeclaration or not.
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*
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* @param {ASTNode} identifier - An Identifier node to check.
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* @returns {boolean} `true` if the node can become a VariableDeclaration.
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*/
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function canBecomeVariableDeclaration(identifier) {
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let node = identifier.parent;
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while (PATTERN_TYPE.test(node.type)) {
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node = node.parent;
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}
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return (
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node.type === "VariableDeclarator" ||
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(
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node.type === "AssignmentExpression" &&
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node.parent.type === "ExpressionStatement" &&
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DECLARATION_HOST_TYPE.test(node.parent.parent.type)
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)
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);
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}
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/**
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* Gets an identifier node of a given variable.
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*
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* If the initialization exists or one or more reading references exist before
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* the first assignment, the identifier node is the node of the declaration.
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* Otherwise, the identifier node is the node of the first assignment.
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*
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* If the variable should not change to const, this function returns null.
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* - If the variable is reassigned.
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* - If the variable is never initialized nor assigned.
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* - If the variable is initialized in a different scope from the declaration.
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* - If the unique assignment of the variable cannot change to a declaration.
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* e.g. `if (a) b = 1` / `return (b = 1)`
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* - If the variable is declared in the global scope and `eslintUsed` is `true`.
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* `/*exported foo` directive comment makes such variables. This rule does not
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* warn such variables because this rule cannot distinguish whether the
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* exported variables are reassigned or not.
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*
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* @param {eslint-scope.Variable} variable - A variable to get.
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* @param {boolean} ignoreReadBeforeAssign -
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* The value of `ignoreReadBeforeAssign` option.
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* @returns {ASTNode|null}
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* An Identifier node if the variable should change to const.
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* Otherwise, null.
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*/
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function getIdentifierIfShouldBeConst(variable, ignoreReadBeforeAssign) {
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if (variable.eslintUsed && variable.scope.type === "global") {
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return null;
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}
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// Finds the unique WriteReference.
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let writer = null;
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let isReadBeforeInit = false;
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const references = variable.references;
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for (let i = 0; i < references.length; ++i) {
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const reference = references[i];
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if (reference.isWrite()) {
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const isReassigned = (
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writer !== null &&
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writer.identifier !== reference.identifier
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);
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if (isReassigned) {
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return null;
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}
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writer = reference;
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} else if (reference.isRead() && writer === null) {
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if (ignoreReadBeforeAssign) {
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return null;
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}
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isReadBeforeInit = true;
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}
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}
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/*
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* If the assignment is from a different scope, ignore it.
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* If the assignment cannot change to a declaration, ignore it.
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*/
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const shouldBeConst = (
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writer !== null &&
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writer.from === variable.scope &&
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canBecomeVariableDeclaration(writer.identifier)
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);
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if (!shouldBeConst) {
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return null;
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}
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if (isReadBeforeInit) {
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return variable.defs[0].name;
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}
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return writer.identifier;
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}
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/**
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* Gets the VariableDeclarator/AssignmentExpression node that a given reference
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* belongs to.
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* This is used to detect a mix of reassigned and never reassigned in a
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* destructuring.
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*
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* @param {eslint-scope.Reference} reference - A reference to get.
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* @returns {ASTNode|null} A VariableDeclarator/AssignmentExpression node or
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* null.
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*/
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function getDestructuringHost(reference) {
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if (!reference.isWrite()) {
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return null;
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}
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let node = reference.identifier.parent;
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while (PATTERN_TYPE.test(node.type)) {
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node = node.parent;
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}
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if (!DESTRUCTURING_HOST_TYPE.test(node.type)) {
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return null;
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}
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return node;
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}
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/**
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* Groups by the VariableDeclarator/AssignmentExpression node that each
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* reference of given variables belongs to.
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* This is used to detect a mix of reassigned and never reassigned in a
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* destructuring.
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*
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* @param {eslint-scope.Variable[]} variables - Variables to group by destructuring.
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* @param {boolean} ignoreReadBeforeAssign -
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* The value of `ignoreReadBeforeAssign` option.
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* @returns {Map<ASTNode, ASTNode[]>} Grouped identifier nodes.
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*/
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function groupByDestructuring(variables, ignoreReadBeforeAssign) {
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const identifierMap = new Map();
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for (let i = 0; i < variables.length; ++i) {
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const variable = variables[i];
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const references = variable.references;
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const identifier = getIdentifierIfShouldBeConst(variable, ignoreReadBeforeAssign);
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let prevId = null;
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for (let j = 0; j < references.length; ++j) {
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const reference = references[j];
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const id = reference.identifier;
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/*
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* Avoid counting a reference twice or more for default values of
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* destructuring.
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*/
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if (id === prevId) {
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continue;
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}
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prevId = id;
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// Add the identifier node into the destructuring group.
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const group = getDestructuringHost(reference);
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if (group) {
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if (identifierMap.has(group)) {
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identifierMap.get(group).push(identifier);
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} else {
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identifierMap.set(group, [identifier]);
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}
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}
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}
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}
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return identifierMap;
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}
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/**
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* Finds the nearest parent of node with a given type.
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*
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* @param {ASTNode} node – The node to search from.
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* @param {string} type – The type field of the parent node.
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* @param {Function} shouldStop – a predicate that returns true if the traversal should stop, and false otherwise.
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* @returns {ASTNode} The closest ancestor with the specified type; null if no such ancestor exists.
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*/
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function findUp(node, type, shouldStop) {
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if (!node || shouldStop(node)) {
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return null;
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}
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if (node.type === type) {
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return node;
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}
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return findUp(node.parent, type, shouldStop);
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}
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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module.exports = {
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meta: {
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docs: {
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description: "require `const` declarations for variables that are never reassigned after declared",
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category: "ECMAScript 6",
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recommended: false,
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url: "https://eslint.org/docs/rules/prefer-const"
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},
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fixable: "code",
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schema: [
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{
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type: "object",
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properties: {
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destructuring: { enum: ["any", "all"] },
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ignoreReadBeforeAssign: { type: "boolean" }
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},
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additionalProperties: false
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}
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]
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},
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create(context) {
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const options = context.options[0] || {};
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const sourceCode = context.getSourceCode();
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const checkingMixedDestructuring = options.destructuring !== "all";
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const ignoreReadBeforeAssign = options.ignoreReadBeforeAssign === true;
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const variables = [];
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/**
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* Reports given identifier nodes if all of the nodes should be declared
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* as const.
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*
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* The argument 'nodes' is an array of Identifier nodes.
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* This node is the result of 'getIdentifierIfShouldBeConst()', so it's
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* nullable. In simple declaration or assignment cases, the length of
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* the array is 1. In destructuring cases, the length of the array can
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* be 2 or more.
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*
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* @param {(eslint-scope.Reference|null)[]} nodes -
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* References which are grouped by destructuring to report.
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* @returns {void}
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*/
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function checkGroup(nodes) {
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const nodesToReport = nodes.filter(Boolean);
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if (nodes.length && (checkingMixedDestructuring || nodesToReport.length === nodes.length)) {
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const varDeclParent = findUp(nodes[0], "VariableDeclaration", parentNode => parentNode.type.endsWith("Statement"));
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const shouldFix = varDeclParent &&
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/*
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* If there are multiple variable declarations, like {let a = 1, b = 2}, then
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* do not attempt to fix if one of the declarations should be `const`. It's
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* too hard to know how the developer would want to automatically resolve the issue.
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*/
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varDeclParent.declarations.length === 1 &&
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// 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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(varDeclParent.parent.type === "ForInStatement" || varDeclParent.parent.type === "ForOfStatement" || varDeclParent.declarations[0].init) &&
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/*
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* If options.destucturing is "all", then this warning will not occur unless
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* every assignment in the destructuring should be const. In that case, it's safe
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* to apply the fix.
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*/
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nodesToReport.length === nodes.length;
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nodesToReport.forEach(node => {
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context.report({
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node,
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message: "'{{name}}' is never reassigned. Use 'const' instead.",
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data: node,
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fix: shouldFix ? fixer => fixer.replaceText(sourceCode.getFirstToken(varDeclParent), "const") : null
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});
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});
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}
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}
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return {
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"Program:exit"() {
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groupByDestructuring(variables, ignoreReadBeforeAssign).forEach(checkGroup);
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},
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VariableDeclaration(node) {
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if (node.kind === "let" && !isInitOfForStatement(node)) {
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pushAll(variables, context.getDeclaredVariables(node));
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}
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}
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};
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}
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};
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