2 * @fileoverview Rule to check for the usage of var.
3 * @author Jamund Ferguson
8 //------------------------------------------------------------------------------
10 //------------------------------------------------------------------------------
12 const astUtils = require("./utils/ast-utils");
14 //------------------------------------------------------------------------------
16 //------------------------------------------------------------------------------
19 * Check whether a given variable is a global variable or not.
20 * @param {eslint-scope.Variable} variable The variable to check.
21 * @returns {boolean} `true` if the variable is a global variable.
23 function isGlobal(variable) {
24 return Boolean(variable.scope) && variable.scope.type === "global";
28 * Finds the nearest function scope or global scope walking up the scope
30 * @param {eslint-scope.Scope} scope The scope to traverse.
31 * @returns {eslint-scope.Scope} a function scope or global scope containing the given
34 function getEnclosingFunctionScope(scope) {
35 let currentScope = scope;
37 while (currentScope.type !== "function" && currentScope.type !== "global") {
38 currentScope = currentScope.upper;
44 * Checks whether the given variable has any references from a more specific
45 * function expression (i.e. a closure).
46 * @param {eslint-scope.Variable} variable A variable to check.
47 * @returns {boolean} `true` if the variable is used from a closure.
49 function isReferencedInClosure(variable) {
50 const enclosingFunctionScope = getEnclosingFunctionScope(variable.scope);
52 return variable.references.some(reference =>
53 getEnclosingFunctionScope(reference.from) !== enclosingFunctionScope);
57 * Checks whether the given node is the assignee of a loop.
58 * @param {ASTNode} node A VariableDeclaration node to check.
59 * @returns {boolean} `true` if the declaration is assigned as part of loop
62 function isLoopAssignee(node) {
63 return (node.parent.type === "ForOfStatement" || node.parent.type === "ForInStatement") &&
64 node === node.parent.left;
68 * Checks whether the given variable declaration is immediately initialized.
69 * @param {ASTNode} node A VariableDeclaration node to check.
70 * @returns {boolean} `true` if the declaration has an initializer.
72 function isDeclarationInitialized(node) {
73 return node.declarations.every(declarator => declarator.init !== null);
76 const SCOPE_NODE_TYPE = /^(?:Program|BlockStatement|SwitchStatement|ForStatement|ForInStatement|ForOfStatement)$/u;
79 * Gets the scope node which directly contains a given node.
80 * @param {ASTNode} node A node to get. This is a `VariableDeclaration` or
82 * @returns {ASTNode} A scope node. This is one of `Program`, `BlockStatement`,
83 * `SwitchStatement`, `ForStatement`, `ForInStatement`, and
86 function getScopeNode(node) {
87 for (let currentNode = node; currentNode; currentNode = currentNode.parent) {
88 if (SCOPE_NODE_TYPE.test(currentNode.type)) {
93 /* istanbul ignore next : unreachable */
98 * Checks whether a given variable is redeclared or not.
99 * @param {eslint-scope.Variable} variable A variable to check.
100 * @returns {boolean} `true` if the variable is redeclared.
102 function isRedeclared(variable) {
103 return variable.defs.length >= 2;
107 * Checks whether a given variable is used from outside of the specified scope.
108 * @param {ASTNode} scopeNode A scope node to check.
109 * @returns {Function} The predicate function which checks whether a given
110 * variable is used from outside of the specified scope.
112 function isUsedFromOutsideOf(scopeNode) {
115 * Checks whether a given reference is inside of the specified scope or not.
116 * @param {eslint-scope.Reference} reference A reference to check.
117 * @returns {boolean} `true` if the reference is inside of the specified
120 function isOutsideOfScope(reference) {
121 const scope = scopeNode.range;
122 const id = reference.identifier.range;
124 return id[0] < scope[0] || id[1] > scope[1];
127 return function(variable) {
128 return variable.references.some(isOutsideOfScope);
133 * Creates the predicate function which checks whether a variable has their references in TDZ.
135 * The predicate function would return `true`:
137 * - if a reference is before the declarator. E.g. (var a = b, b = 1;)(var {a = b, b} = {};)
138 * - if a reference is in the expression of their default value. E.g. (var {a = a} = {};)
139 * - if a reference is in the expression of their initializer. E.g. (var a = a;)
140 * @param {ASTNode} node The initializer node of VariableDeclarator.
141 * @returns {Function} The predicate function.
144 function hasReferenceInTDZ(node) {
145 const initStart = node.range[0];
146 const initEnd = node.range[1];
149 const id = variable.defs[0].name;
150 const idStart = id.range[0];
151 const defaultValue = (id.parent.type === "AssignmentPattern" ? id.parent.right : null);
152 const defaultStart = defaultValue && defaultValue.range[0];
153 const defaultEnd = defaultValue && defaultValue.range[1];
155 return variable.references.some(reference => {
156 const start = reference.identifier.range[0];
157 const end = reference.identifier.range[1];
159 return !reference.init && (
161 (defaultValue !== null && start >= defaultStart && end <= defaultEnd) ||
162 (start >= initStart && end <= initEnd)
169 * Checks whether a given variable has name that is allowed for 'var' declarations,
170 * but disallowed for `let` declarations.
171 * @param {eslint-scope.Variable} variable The variable to check.
172 * @returns {boolean} `true` if the variable has a disallowed name.
174 function hasNameDisallowedForLetDeclarations(variable) {
175 return variable.name === "let";
178 //------------------------------------------------------------------------------
180 //------------------------------------------------------------------------------
187 description: "require `let` or `const` instead of `var`",
188 category: "ECMAScript 6",
190 url: "https://eslint.org/docs/rules/no-var"
197 unexpectedVar: "Unexpected var, use let or const instead."
202 const sourceCode = context.getSourceCode();
205 * Checks whether the variables which are defined by the given declarator node have their references in TDZ.
206 * @param {ASTNode} declarator The VariableDeclarator node to check.
207 * @returns {boolean} `true` if one of the variables which are defined by the given declarator node have their references in TDZ.
209 function hasSelfReferenceInTDZ(declarator) {
210 if (!declarator.init) {
213 const variables = context.getDeclaredVariables(declarator);
215 return variables.some(hasReferenceInTDZ(declarator.init));
219 * Checks whether it can fix a given variable declaration or not.
220 * It cannot fix if the following cases:
222 * - A variable is a global variable.
223 * - A variable is declared on a SwitchCase node.
224 * - A variable is redeclared.
225 * - A variable is used from outside the scope.
226 * - A variable is used from a closure within a loop.
227 * - A variable might be used before it is assigned within a loop.
228 * - A variable might be used in TDZ.
229 * - A variable is declared in statement position (e.g. a single-line `IfStatement`)
230 * - A variable has name that is disallowed for `let` declarations.
232 * ## A variable is declared on a SwitchCase node.
234 * If this rule modifies 'var' declarations on a SwitchCase node, it
235 * would generate the warnings of 'no-case-declarations' rule. And the
236 * 'eslint:recommended' preset includes 'no-case-declarations' rule, so
237 * this rule doesn't modify those declarations.
239 * ## A variable is redeclared.
241 * The language spec disallows redeclarations of `let` declarations.
242 * Those variables would cause syntax errors.
244 * ## A variable is used from outside the scope.
246 * The language spec disallows accesses from outside of the scope for
247 * `let` declarations. Those variables would cause reference errors.
249 * ## A variable is used from a closure within a loop.
251 * A `var` declaration within a loop shares the same variable instance
252 * across all loop iterations, while a `let` declaration creates a new
253 * instance for each iteration. This means if a variable in a loop is
254 * referenced by any closure, changing it from `var` to `let` would
255 * change the behavior in a way that is generally unsafe.
257 * ## A variable might be used before it is assigned within a loop.
259 * Within a loop, a `let` declaration without an initializer will be
260 * initialized to null, while a `var` declaration will retain its value
261 * from the previous iteration, so it is only safe to change `var` to
262 * `let` if we can statically determine that the variable is always
263 * assigned a value before its first access in the loop body. To keep
264 * the implementation simple, we only convert `var` to `let` within
265 * loops when the variable is a loop assignee or the declaration has an
267 * @param {ASTNode} node A variable declaration node to check.
268 * @returns {boolean} `true` if it can fix the node.
270 function canFix(node) {
271 const variables = context.getDeclaredVariables(node);
272 const scopeNode = getScopeNode(node);
274 if (node.parent.type === "SwitchCase" ||
275 node.declarations.some(hasSelfReferenceInTDZ) ||
276 variables.some(isGlobal) ||
277 variables.some(isRedeclared) ||
278 variables.some(isUsedFromOutsideOf(scopeNode)) ||
279 variables.some(hasNameDisallowedForLetDeclarations)
284 if (astUtils.isInLoop(node)) {
285 if (variables.some(isReferencedInClosure)) {
288 if (!isLoopAssignee(node) && !isDeclarationInitialized(node)) {
294 !isLoopAssignee(node) &&
295 !(node.parent.type === "ForStatement" && node.parent.init === node) &&
296 !astUtils.STATEMENT_LIST_PARENTS.has(node.parent.type)
299 // If the declaration is not in a block, e.g. `if (foo) var bar = 1;`, then it can't be fixed.
307 * Reports a given variable declaration node.
308 * @param {ASTNode} node A variable declaration node to report.
311 function report(node) {
314 messageId: "unexpectedVar",
317 const varToken = sourceCode.getFirstToken(node, { filter: t => t.value === "var" });
320 ? fixer.replaceText(varToken, "let")
327 "VariableDeclaration:exit"(node) {
328 if (node.kind === "var") {