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335 lines
12 KiB
335 lines
12 KiB
/**
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* @fileoverview Rule to check for the usage of var.
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* @author Jamund Ferguson
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*/
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"use strict";
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//------------------------------------------------------------------------------
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// Requirements
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//------------------------------------------------------------------------------
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const astUtils = require("./utils/ast-utils");
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//------------------------------------------------------------------------------
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// Helpers
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//------------------------------------------------------------------------------
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/**
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* Check whether a given variable is a global variable or not.
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* @param {eslint-scope.Variable} variable The variable to check.
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* @returns {boolean} `true` if the variable is a global variable.
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*/
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function isGlobal(variable) {
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return Boolean(variable.scope) && variable.scope.type === "global";
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}
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/**
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* Finds the nearest function scope or global scope walking up the scope
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* hierarchy.
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* @param {eslint-scope.Scope} scope The scope to traverse.
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* @returns {eslint-scope.Scope} a function scope or global scope containing the given
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* scope.
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*/
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function getEnclosingFunctionScope(scope) {
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let currentScope = scope;
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while (currentScope.type !== "function" && currentScope.type !== "global") {
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currentScope = currentScope.upper;
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}
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return currentScope;
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}
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/**
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* Checks whether the given variable has any references from a more specific
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* function expression (i.e. a closure).
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* @param {eslint-scope.Variable} variable A variable to check.
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* @returns {boolean} `true` if the variable is used from a closure.
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*/
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function isReferencedInClosure(variable) {
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const enclosingFunctionScope = getEnclosingFunctionScope(variable.scope);
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return variable.references.some(reference =>
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getEnclosingFunctionScope(reference.from) !== enclosingFunctionScope);
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}
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/**
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* Checks whether the given node is the assignee of a loop.
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* @param {ASTNode} node A VariableDeclaration node to check.
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* @returns {boolean} `true` if the declaration is assigned as part of loop
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* iteration.
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*/
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function isLoopAssignee(node) {
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return (node.parent.type === "ForOfStatement" || node.parent.type === "ForInStatement") &&
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node === node.parent.left;
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}
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/**
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* Checks whether the given variable declaration is immediately initialized.
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* @param {ASTNode} node A VariableDeclaration node to check.
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* @returns {boolean} `true` if the declaration has an initializer.
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*/
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function isDeclarationInitialized(node) {
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return node.declarations.every(declarator => declarator.init !== null);
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}
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const SCOPE_NODE_TYPE = /^(?:Program|BlockStatement|SwitchStatement|ForStatement|ForInStatement|ForOfStatement)$/u;
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/**
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* Gets the scope node which directly contains a given node.
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* @param {ASTNode} node A node to get. This is a `VariableDeclaration` or
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* an `Identifier`.
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* @returns {ASTNode} A scope node. This is one of `Program`, `BlockStatement`,
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* `SwitchStatement`, `ForStatement`, `ForInStatement`, and
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* `ForOfStatement`.
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*/
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function getScopeNode(node) {
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for (let currentNode = node; currentNode; currentNode = currentNode.parent) {
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if (SCOPE_NODE_TYPE.test(currentNode.type)) {
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return currentNode;
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}
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}
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/* c8 ignore next */
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return null;
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}
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/**
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* Checks whether a given variable is redeclared or not.
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* @param {eslint-scope.Variable} variable A variable to check.
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* @returns {boolean} `true` if the variable is redeclared.
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*/
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function isRedeclared(variable) {
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return variable.defs.length >= 2;
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}
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/**
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* Checks whether a given variable is used from outside of the specified scope.
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* @param {ASTNode} scopeNode A scope node to check.
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* @returns {Function} The predicate function which checks whether a given
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* variable is used from outside of the specified scope.
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*/
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function isUsedFromOutsideOf(scopeNode) {
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/**
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* Checks whether a given reference is inside of the specified scope or not.
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* @param {eslint-scope.Reference} reference A reference to check.
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* @returns {boolean} `true` if the reference is inside of the specified
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* scope.
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*/
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function isOutsideOfScope(reference) {
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const scope = scopeNode.range;
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const id = reference.identifier.range;
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return id[0] < scope[0] || id[1] > scope[1];
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}
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return function(variable) {
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return variable.references.some(isOutsideOfScope);
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};
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}
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/**
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* Creates the predicate function which checks whether a variable has their references in TDZ.
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*
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* The predicate function would return `true`:
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*
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* - if a reference is before the declarator. E.g. (var a = b, b = 1;)(var {a = b, b} = {};)
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* - if a reference is in the expression of their default value. E.g. (var {a = a} = {};)
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* - if a reference is in the expression of their initializer. E.g. (var a = a;)
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* @param {ASTNode} node The initializer node of VariableDeclarator.
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* @returns {Function} The predicate function.
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* @private
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*/
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function hasReferenceInTDZ(node) {
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const initStart = node.range[0];
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const initEnd = node.range[1];
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return variable => {
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const id = variable.defs[0].name;
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const idStart = id.range[0];
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const defaultValue = (id.parent.type === "AssignmentPattern" ? id.parent.right : null);
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const defaultStart = defaultValue && defaultValue.range[0];
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const defaultEnd = defaultValue && defaultValue.range[1];
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return variable.references.some(reference => {
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const start = reference.identifier.range[0];
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const end = reference.identifier.range[1];
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return !reference.init && (
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start < idStart ||
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(defaultValue !== null && start >= defaultStart && end <= defaultEnd) ||
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(!astUtils.isFunction(node) && start >= initStart && end <= initEnd)
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);
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});
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};
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}
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/**
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* Checks whether a given variable has name that is allowed for 'var' declarations,
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* but disallowed for `let` declarations.
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* @param {eslint-scope.Variable} variable The variable to check.
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* @returns {boolean} `true` if the variable has a disallowed name.
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*/
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function hasNameDisallowedForLetDeclarations(variable) {
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return variable.name === "let";
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}
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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/** @type {import('../shared/types').Rule} */
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module.exports = {
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meta: {
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type: "suggestion",
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docs: {
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description: "Require `let` or `const` instead of `var`",
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recommended: false,
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url: "https://eslint.org/docs/latest/rules/no-var"
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},
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schema: [],
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fixable: "code",
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messages: {
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unexpectedVar: "Unexpected var, use let or const instead."
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}
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},
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create(context) {
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const sourceCode = context.sourceCode;
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/**
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* Checks whether the variables which are defined by the given declarator node have their references in TDZ.
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* @param {ASTNode} declarator The VariableDeclarator node to check.
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* @returns {boolean} `true` if one of the variables which are defined by the given declarator node have their references in TDZ.
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*/
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function hasSelfReferenceInTDZ(declarator) {
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if (!declarator.init) {
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return false;
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}
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const variables = sourceCode.getDeclaredVariables(declarator);
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return variables.some(hasReferenceInTDZ(declarator.init));
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}
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/**
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* Checks whether it can fix a given variable declaration or not.
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* It cannot fix if the following cases:
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*
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* - A variable is a global variable.
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* - A variable is declared on a SwitchCase node.
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* - A variable is redeclared.
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* - A variable is used from outside the scope.
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* - A variable is used from a closure within a loop.
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* - A variable might be used before it is assigned within a loop.
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* - A variable might be used in TDZ.
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* - A variable is declared in statement position (e.g. a single-line `IfStatement`)
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* - A variable has name that is disallowed for `let` declarations.
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*
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* ## A variable is declared on a SwitchCase node.
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*
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* If this rule modifies 'var' declarations on a SwitchCase node, it
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* would generate the warnings of 'no-case-declarations' rule. And the
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* 'eslint:recommended' preset includes 'no-case-declarations' rule, so
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* this rule doesn't modify those declarations.
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*
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* ## A variable is redeclared.
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*
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* The language spec disallows redeclarations of `let` declarations.
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* Those variables would cause syntax errors.
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*
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* ## A variable is used from outside the scope.
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*
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* The language spec disallows accesses from outside of the scope for
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* `let` declarations. Those variables would cause reference errors.
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*
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* ## A variable is used from a closure within a loop.
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*
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* A `var` declaration within a loop shares the same variable instance
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* across all loop iterations, while a `let` declaration creates a new
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* instance for each iteration. This means if a variable in a loop is
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* referenced by any closure, changing it from `var` to `let` would
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* change the behavior in a way that is generally unsafe.
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*
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* ## A variable might be used before it is assigned within a loop.
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*
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* Within a loop, a `let` declaration without an initializer will be
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* initialized to null, while a `var` declaration will retain its value
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* from the previous iteration, so it is only safe to change `var` to
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* `let` if we can statically determine that the variable is always
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* assigned a value before its first access in the loop body. To keep
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* the implementation simple, we only convert `var` to `let` within
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* loops when the variable is a loop assignee or the declaration has an
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* initializer.
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* @param {ASTNode} node A variable declaration node to check.
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* @returns {boolean} `true` if it can fix the node.
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*/
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function canFix(node) {
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const variables = sourceCode.getDeclaredVariables(node);
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const scopeNode = getScopeNode(node);
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if (node.parent.type === "SwitchCase" ||
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node.declarations.some(hasSelfReferenceInTDZ) ||
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variables.some(isGlobal) ||
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variables.some(isRedeclared) ||
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variables.some(isUsedFromOutsideOf(scopeNode)) ||
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variables.some(hasNameDisallowedForLetDeclarations)
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) {
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return false;
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}
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if (astUtils.isInLoop(node)) {
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if (variables.some(isReferencedInClosure)) {
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return false;
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}
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if (!isLoopAssignee(node) && !isDeclarationInitialized(node)) {
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return false;
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}
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}
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if (
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!isLoopAssignee(node) &&
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!(node.parent.type === "ForStatement" && node.parent.init === node) &&
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!astUtils.STATEMENT_LIST_PARENTS.has(node.parent.type)
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) {
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// If the declaration is not in a block, e.g. `if (foo) var bar = 1;`, then it can't be fixed.
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return false;
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}
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return true;
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}
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/**
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* Reports a given variable declaration node.
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* @param {ASTNode} node A variable declaration node to report.
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* @returns {void}
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*/
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function report(node) {
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context.report({
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node,
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messageId: "unexpectedVar",
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fix(fixer) {
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const varToken = sourceCode.getFirstToken(node, { filter: t => t.value === "var" });
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return canFix(node)
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? fixer.replaceText(varToken, "let")
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: null;
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}
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});
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}
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return {
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"VariableDeclaration:exit"(node) {
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if (node.kind === "var") {
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report(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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