1 | /**
|
2 | * @fileoverview Rule to check for the usage of var.
|
3 | * @author Jamund Ferguson
|
4 | */
|
5 |
|
6 | ;
|
7 |
|
8 | //------------------------------------------------------------------------------
|
9 | // Requirements
|
10 | //------------------------------------------------------------------------------
|
11 |
|
12 | const astUtils = require("../ast-utils");
|
13 |
|
14 | //------------------------------------------------------------------------------
|
15 | // Helpers
|
16 | //------------------------------------------------------------------------------
|
17 |
|
18 | /**
|
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.
|
22 | */
|
23 | function isGlobal(variable) {
|
24 | return Boolean(variable.scope) && variable.scope.type === "global";
|
25 | }
|
26 |
|
27 | /**
|
28 | * Finds the nearest function scope or global scope walking up the scope
|
29 | * hierarchy.
|
30 | *
|
31 | * @param {eslint-scope.Scope} scope - The scope to traverse.
|
32 | * @returns {eslint-scope.Scope} a function scope or global scope containing the given
|
33 | * scope.
|
34 | */
|
35 | function getEnclosingFunctionScope(scope) {
|
36 | while (scope.type !== "function" && scope.type !== "global") {
|
37 | scope = scope.upper;
|
38 | }
|
39 | return scope;
|
40 | }
|
41 |
|
42 | /**
|
43 | * Checks whether the given variable has any references from a more specific
|
44 | * function expression (i.e. a closure).
|
45 | *
|
46 | * @param {eslint-scope.Variable} variable - A variable to check.
|
47 | * @returns {boolean} `true` if the variable is used from a closure.
|
48 | */
|
49 | function isReferencedInClosure(variable) {
|
50 | const enclosingFunctionScope = getEnclosingFunctionScope(variable.scope);
|
51 |
|
52 | return variable.references.some(reference =>
|
53 | getEnclosingFunctionScope(reference.from) !== enclosingFunctionScope);
|
54 | }
|
55 |
|
56 | /**
|
57 | * Checks whether the given node is the assignee of a loop.
|
58 | *
|
59 | * @param {ASTNode} node - A VariableDeclaration node to check.
|
60 | * @returns {boolean} `true` if the declaration is assigned as part of loop
|
61 | * iteration.
|
62 | */
|
63 | function isLoopAssignee(node) {
|
64 | return (node.parent.type === "ForOfStatement" || node.parent.type === "ForInStatement") &&
|
65 | node === node.parent.left;
|
66 | }
|
67 |
|
68 | /**
|
69 | * Checks whether the given variable declaration is immediately initialized.
|
70 | *
|
71 | * @param {ASTNode} node - A VariableDeclaration node to check.
|
72 | * @returns {boolean} `true` if the declaration has an initializer.
|
73 | */
|
74 | function isDeclarationInitialized(node) {
|
75 | return node.declarations.every(declarator => declarator.init !== null);
|
76 | }
|
77 |
|
78 | const SCOPE_NODE_TYPE = /^(?:Program|BlockStatement|SwitchStatement|ForStatement|ForInStatement|ForOfStatement)$/;
|
79 |
|
80 | /**
|
81 | * Gets the scope node which directly contains a given node.
|
82 | *
|
83 | * @param {ASTNode} node - A node to get. This is a `VariableDeclaration` or
|
84 | * an `Identifier`.
|
85 | * @returns {ASTNode} A scope node. This is one of `Program`, `BlockStatement`,
|
86 | * `SwitchStatement`, `ForStatement`, `ForInStatement`, and
|
87 | * `ForOfStatement`.
|
88 | */
|
89 | function getScopeNode(node) {
|
90 | while (node) {
|
91 | if (SCOPE_NODE_TYPE.test(node.type)) {
|
92 | return node;
|
93 | }
|
94 |
|
95 | node = node.parent;
|
96 | }
|
97 |
|
98 | /* istanbul ignore next : unreachable */
|
99 | return null;
|
100 | }
|
101 |
|
102 | /**
|
103 | * Checks whether a given variable is redeclared or not.
|
104 | *
|
105 | * @param {eslint-scope.Variable} variable - A variable to check.
|
106 | * @returns {boolean} `true` if the variable is redeclared.
|
107 | */
|
108 | function isRedeclared(variable) {
|
109 | return variable.defs.length >= 2;
|
110 | }
|
111 |
|
112 | /**
|
113 | * Checks whether a given variable is used from outside of the specified scope.
|
114 | *
|
115 | * @param {ASTNode} scopeNode - A scope node to check.
|
116 | * @returns {Function} The predicate function which checks whether a given
|
117 | * variable is used from outside of the specified scope.
|
118 | */
|
119 | function isUsedFromOutsideOf(scopeNode) {
|
120 |
|
121 | /**
|
122 | * Checks whether a given reference is inside of the specified scope or not.
|
123 | *
|
124 | * @param {eslint-scope.Reference} reference - A reference to check.
|
125 | * @returns {boolean} `true` if the reference is inside of the specified
|
126 | * scope.
|
127 | */
|
128 | function isOutsideOfScope(reference) {
|
129 | const scope = scopeNode.range;
|
130 | const id = reference.identifier.range;
|
131 |
|
132 | return id[0] < scope[0] || id[1] > scope[1];
|
133 | }
|
134 |
|
135 | return function(variable) {
|
136 | return variable.references.some(isOutsideOfScope);
|
137 | };
|
138 | }
|
139 |
|
140 | /**
|
141 | * Creates the predicate function which checks whether a variable has their references in TDZ.
|
142 | *
|
143 | * The predicate function would return `true`:
|
144 | *
|
145 | * - if a reference is before the declarator. E.g. (var a = b, b = 1;)(var {a = b, b} = {};)
|
146 | * - if a reference is in the expression of their default value. E.g. (var {a = a} = {};)
|
147 | * - if a reference is in the expression of their initializer. E.g. (var a = a;)
|
148 | *
|
149 | * @param {ASTNode} node - The initializer node of VariableDeclarator.
|
150 | * @returns {Function} The predicate function.
|
151 | * @private
|
152 | */
|
153 | function hasReferenceInTDZ(node) {
|
154 | const initStart = node.range[0];
|
155 | const initEnd = node.range[1];
|
156 |
|
157 | return variable => {
|
158 | const id = variable.defs[0].name;
|
159 | const idStart = id.range[0];
|
160 | const defaultValue = (id.parent.type === "AssignmentPattern" ? id.parent.right : null);
|
161 | const defaultStart = defaultValue && defaultValue.range[0];
|
162 | const defaultEnd = defaultValue && defaultValue.range[1];
|
163 |
|
164 | return variable.references.some(reference => {
|
165 | const start = reference.identifier.range[0];
|
166 | const end = reference.identifier.range[1];
|
167 |
|
168 | return !reference.init && (
|
169 | start < idStart ||
|
170 | (defaultValue !== null && start >= defaultStart && end <= defaultEnd) ||
|
171 | (start >= initStart && end <= initEnd)
|
172 | );
|
173 | });
|
174 | };
|
175 | }
|
176 |
|
177 | //------------------------------------------------------------------------------
|
178 | // Rule Definition
|
179 | //------------------------------------------------------------------------------
|
180 |
|
181 | module.exports = {
|
182 | meta: {
|
183 | docs: {
|
184 | description: "require `let` or `const` instead of `var`",
|
185 | category: "ECMAScript 6",
|
186 | recommended: false,
|
187 | url: "https://eslint.org/docs/rules/no-var"
|
188 | },
|
189 |
|
190 | schema: [],
|
191 | fixable: "code"
|
192 | },
|
193 |
|
194 | create(context) {
|
195 | const sourceCode = context.getSourceCode();
|
196 |
|
197 | /**
|
198 | * Checks whether the variables which are defined by the given declarator node have their references in TDZ.
|
199 | *
|
200 | * @param {ASTNode} declarator - The VariableDeclarator node to check.
|
201 | * @returns {boolean} `true` if one of the variables which are defined by the given declarator node have their references in TDZ.
|
202 | */
|
203 | function hasSelfReferenceInTDZ(declarator) {
|
204 | if (!declarator.init) {
|
205 | return false;
|
206 | }
|
207 | const variables = context.getDeclaredVariables(declarator);
|
208 |
|
209 | return variables.some(hasReferenceInTDZ(declarator.init));
|
210 | }
|
211 |
|
212 | /**
|
213 | * Checks whether it can fix a given variable declaration or not.
|
214 | * It cannot fix if the following cases:
|
215 | *
|
216 | * - A variable is a global variable.
|
217 | * - A variable is declared on a SwitchCase node.
|
218 | * - A variable is redeclared.
|
219 | * - A variable is used from outside the scope.
|
220 | * - A variable is used from a closure within a loop.
|
221 | * - A variable might be used before it is assigned within a loop.
|
222 | * - A variable might be used in TDZ.
|
223 | * - A variable is declared in statement position (e.g. a single-line `IfStatement`)
|
224 | *
|
225 | * ## A variable is declared on a SwitchCase node.
|
226 | *
|
227 | * If this rule modifies 'var' declarations on a SwitchCase node, it
|
228 | * would generate the warnings of 'no-case-declarations' rule. And the
|
229 | * 'eslint:recommended' preset includes 'no-case-declarations' rule, so
|
230 | * this rule doesn't modify those declarations.
|
231 | *
|
232 | * ## A variable is redeclared.
|
233 | *
|
234 | * The language spec disallows redeclarations of `let` declarations.
|
235 | * Those variables would cause syntax errors.
|
236 | *
|
237 | * ## A variable is used from outside the scope.
|
238 | *
|
239 | * The language spec disallows accesses from outside of the scope for
|
240 | * `let` declarations. Those variables would cause reference errors.
|
241 | *
|
242 | * ## A variable is used from a closure within a loop.
|
243 | *
|
244 | * A `var` declaration within a loop shares the same variable instance
|
245 | * across all loop iterations, while a `let` declaration creates a new
|
246 | * instance for each iteration. This means if a variable in a loop is
|
247 | * referenced by any closure, changing it from `var` to `let` would
|
248 | * change the behavior in a way that is generally unsafe.
|
249 | *
|
250 | * ## A variable might be used before it is assigned within a loop.
|
251 | *
|
252 | * Within a loop, a `let` declaration without an initializer will be
|
253 | * initialized to null, while a `var` declaration will retain its value
|
254 | * from the previous iteration, so it is only safe to change `var` to
|
255 | * `let` if we can statically determine that the variable is always
|
256 | * assigned a value before its first access in the loop body. To keep
|
257 | * the implementation simple, we only convert `var` to `let` within
|
258 | * loops when the variable is a loop assignee or the declaration has an
|
259 | * initializer.
|
260 | *
|
261 | * @param {ASTNode} node - A variable declaration node to check.
|
262 | * @returns {boolean} `true` if it can fix the node.
|
263 | */
|
264 | function canFix(node) {
|
265 | const variables = context.getDeclaredVariables(node);
|
266 | const scopeNode = getScopeNode(node);
|
267 |
|
268 | if (node.parent.type === "SwitchCase" ||
|
269 | node.declarations.some(hasSelfReferenceInTDZ) ||
|
270 | variables.some(isGlobal) ||
|
271 | variables.some(isRedeclared) ||
|
272 | variables.some(isUsedFromOutsideOf(scopeNode))
|
273 | ) {
|
274 | return false;
|
275 | }
|
276 |
|
277 | if (astUtils.isInLoop(node)) {
|
278 | if (variables.some(isReferencedInClosure)) {
|
279 | return false;
|
280 | }
|
281 | if (!isLoopAssignee(node) && !isDeclarationInitialized(node)) {
|
282 | return false;
|
283 | }
|
284 | }
|
285 |
|
286 | if (
|
287 | !isLoopAssignee(node) &&
|
288 | !(node.parent.type === "ForStatement" && node.parent.init === node) &&
|
289 | !astUtils.STATEMENT_LIST_PARENTS.has(node.parent.type)
|
290 | ) {
|
291 |
|
292 | // If the declaration is not in a block, e.g. `if (foo) var bar = 1;`, then it can't be fixed.
|
293 | return false;
|
294 | }
|
295 |
|
296 | return true;
|
297 | }
|
298 |
|
299 | /**
|
300 | * Reports a given variable declaration node.
|
301 | *
|
302 | * @param {ASTNode} node - A variable declaration node to report.
|
303 | * @returns {void}
|
304 | */
|
305 | function report(node) {
|
306 | const varToken = sourceCode.getFirstToken(node);
|
307 |
|
308 | context.report({
|
309 | node,
|
310 | message: "Unexpected var, use let or const instead.",
|
311 |
|
312 | fix(fixer) {
|
313 | if (canFix(node)) {
|
314 | return fixer.replaceText(varToken, "let");
|
315 | }
|
316 | return null;
|
317 | }
|
318 | });
|
319 | }
|
320 |
|
321 | return {
|
322 | "VariableDeclaration:exit"(node) {
|
323 | if (node.kind === "var") {
|
324 | report(node);
|
325 | }
|
326 | }
|
327 | };
|
328 | }
|
329 | };
|