fix(get-static-value): incorrect return when variable has object mutation (#257)

Author: ota-meshi

src/get-static-value.mjs

@@ -3,6 +3,7 @@
 import { findVariable } from "./find-variable.mjs"
 /** @typedef {import("./types.mjs").StaticValue} StaticValue */
 /** @typedef {import("eslint").Scope.Scope} Scope */
+/** @typedef {import("eslint").Scope.Variable} Variable */
 /** @typedef {import("estree").Node} Node */
 /** @typedef {import("@typescript-eslint/types").TSESTree.Node} TSESTreeNode */
 /** @typedef {import("@typescript-eslint/types").TSESTree.AST_NODE_TYPES} TSESTreeNodeTypes */
@@ -266,9 +267,40 @@ function getElementValues(nodeList, initialScope) {
     return valueList
 }
 
+/**
+ * Checks if a variable is a built-in global.
+ * @param {Variable|null} variable The variable to check.
+ * @returns {variable is Variable & {defs:[]}}
+ */
+function isBuiltinGlobal(variable) {
+    return (
+        variable != null &&
+        variable.defs.length === 0 &&
+        builtinNames.has(variable.name) &&
+        variable.name in globalObject
+    )
+}
+
+/**
+ * Checks if a variable can be considered as a constant.
+ * @param {Variable} variable
+ * @returns {variable is Variable & {defs: [import("eslint").Scope.Definition & { type: "Variable" }]}} True if the variable can be considered as a constant.
+ */
+function canBeConsideredConst(variable) {
+    if (variable.defs.length !== 1) {
+        return false
+    }
+    const def = variable.defs[0]
+    return Boolean(
+        def.parent &&
+            def.type === "Variable" &&
+            (def.parent.kind === "const" || isEffectivelyConst(variable)),
+    )
+}
+
 /**
  * Returns whether the given variable is never written to after initialization.
- * @param {import("eslint").Scope.Variable} variable
+ * @param {Variable} variable
  * @returns {boolean}
  */
 function isEffectivelyConst(variable) {
@@ -283,6 +315,68 @@ function isEffectivelyConst(variable) {
     return false
 }
 
+/**
+ * Checks if a variable has mutation in its property.
+ * @param {Variable} variable The variable to check.
+ * @param {Scope|null} initialScope The scope to start finding variable. Optional. If the node is a computed property node and this scope was given, this checks the computed property name by the `getStringIfConstant` function with the scope, and returns the value of it.
+ * @returns {boolean} True if the variable has mutation in its property.
+ */
+function hasMutationInProperty(variable, initialScope) {
+    for (const ref of variable.references) {
+        let node = /** @type {TSESTreeNode} */ (ref.identifier)
+        while (node && node.parent && node.parent.type === "MemberExpression") {
+            node = node.parent
+        }
+        if (!node || !node.parent) {
+            continue
+        }
+        if (
+            (node.parent.type === "AssignmentExpression" &&
+                node.parent.left === node) ||
+            (node.parent.type === "UpdateExpression" &&
+                node.parent.argument === node)
+        ) {
+            // This is a mutation.
+            return true
+        }
+        if (
+            node.parent.type === "CallExpression" &&
+            node.parent.callee === node &&
+            node.type === "MemberExpression"
+        ) {
+            const methodName = getStaticPropertyNameValue(node, initialScope)
+            if (isNameOfMutationArrayMethod(methodName)) {
+                // This is a mutation.
+                return true
+            }
+        }
+    }
+    return false
+
+    /**
+     * Checks if a method name is one of the mutation array methods.
+     * @param {StaticValue|null} methodName The method name to check.
+     * @returns {boolean} True if the method name is a mutation array method.
+     */
+    function isNameOfMutationArrayMethod(methodName) {
+        if (methodName == null || methodName.value == null) {
+            return false
+        }
+        const name = methodName.value
+        return (
+            name === "copyWithin" ||
+            name === "fill" ||
+            name === "pop" ||
+            name === "push" ||
+            name === "reverse" ||
+            name === "shift" ||
+            name === "sort" ||
+            name === "splice" ||
+            name === "unshift"
+        )
+    }
+}
+
 /**
  * @template {TSESTreeNodeTypes} T
  * @callback VisitorCallback
@@ -512,28 +606,35 @@ const operations = Object.freeze({
         if (initialScope != null) {
             const variable = findVariable(initialScope, node)
 
-            // Built-in globals.
-            if (
-                variable != null &&
-                variable.defs.length === 0 &&
-                builtinNames.has(variable.name) &&
-                variable.name in globalObject
-            ) {
-                return { value: globalObject[variable.name] }
-            }
+            if (variable != null) {
+                // Built-in globals.
+                if (isBuiltinGlobal(variable)) {
+                    return { value: globalObject[variable.name] }
+                }
 
-            // Constants.
-            if (variable != null && variable.defs.length === 1) {
-                const def = variable.defs[0]
-                if (
-                    def.parent &&
-                    def.type === "Variable" &&
-                    (def.parent.kind === "const" ||
-                        isEffectivelyConst(variable)) &&
-                    // TODO(mysticatea): don't support destructuring here.
-                    def.node.id.type === "Identifier"
-                ) {
-                    return getStaticValueR(def.node.init, initialScope)
+                // Constants.
+                if (canBeConsideredConst(variable)) {
+                    const def = variable.defs[0]
+                    if (
+                        // TODO(mysticatea): don't support destructuring here.
+                        def.node.id.type === "Identifier"
+                    ) {
+                        const init = getStaticValueR(
+                            def.node.init,
+                            initialScope,
+                        )
+                        if (
+                            init &&
+                            typeof init.value === "object" &&
+                            init.value !== null
+                        ) {
+                            if (hasMutationInProperty(variable, initialScope)) {
+                                // This variable has mutation in its property.
+                                return null
+                            }
+                        }
+                        return init
+                    }
                 }
             }
         }

test/get-static-value.mjs

@@ -398,6 +398,55 @@ const aMap = Object.freeze({
                   },
               ]
             : []),
+        // Mutations
+        {
+            code: "const a = {foo: 'a'}; a.bar = 'b'; a",
+            expected: null,
+        },
+        {
+            code: "const a = ['a']; a[0] = 'b'; a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a']; a.copyWithin(0, 1); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a']; a.fill('b'); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a']; a.pop(); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a']; a.push('b'); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a', 'b']; a.reverse(); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a']; a.shift(); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['b', 'a', 'c']; a.sort(); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a', 'c']; a.splice(1, 0, 'b'); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = ['a']; a.unshift('b'); a.join()",
+            expected: null,
+        },
+        {
+            code: "const a = {foo: ['a']}; a.foo.shift(); a",
+            expected: null,
+        },
         // TypeScript support
         {
             code: `const a = 42; a as number;`,