feat(getStaticValue): Dereference variables that are effectively const (#80)

Author: RunDevelopment

src/get-static-value.mjs

@@ -249,6 +249,23 @@ function getElementValues(nodeList, initialScope) {
     return valueList
 }
 
+/**
+ * Returns whether the given variable is never written to after initialization.
+ * @param {import("eslint").Scope.Variable} variable
+ * @returns {boolean}
+ */
+function isEffectivelyConst(variable) {
+    const refs = variable.references
+
+    const inits = refs.filter((r) => r.init).length
+    const reads = refs.filter((r) => r.isReadOnly()).length
+    if (inits === 1 && reads + inits === refs.length) {
+        // there is only one init and all other references only read
+        return true
+    }
+    return false
+}
+
 const operations = Object.freeze({
     ArrayExpression(node, initialScope) {
         const elements = getElementValues(node.elements, initialScope)
@@ -407,7 +424,9 @@ const operations = Object.freeze({
                 const def = variable.defs[0]
                 if (
                     def.parent &&
-                    def.parent.kind === "const" &&
+                    def.type === "Variable" &&
+                    (def.parent.kind === "const" ||
+                        isEffectivelyConst(variable)) &&
                     // TODO(mysticatea): don't support destructuring here.
                     def.node.id.type === "Identifier"
                 ) {

test/get-static-value.mjs

@@ -67,7 +67,11 @@ describe("The 'getStaticValue' function", () => {
         { code: "var undefined; undefined", expected: null },
         { code: "const undefined = 1; undefined", expected: { value: 1 } },
         { code: "const a = 2; a", expected: { value: 2 } },
-        { code: "let a = 2; a", expected: null },
+        { code: "let a = 2; a", expected: { value: 2 } },
+        { code: "var a = 2; a", expected: { value: 2 } },
+        { code: "let a = 2; a = 1; a", expected: null },
+        { code: "let a = 2; a++; a", expected: null },
+        { code: "let a; a = 1; a", expected: null },
         { code: "const a = 2; a", expected: null, noScope: true },
         { code: "const a = { b: 7 }; a.b", expected: { value: 7 } },
         { code: "null", expected: { value: null } },
@@ -158,7 +162,14 @@ describe("The 'getStaticValue' function", () => {
             code: "const obj = {b: 2}; ({a: 1, ...obj})",
             expected: { value: { a: 1, b: 2 } },
         },
-        { code: "var obj = {b: 2}; ({a: 1, ...obj})", expected: null },
+        {
+            code: "var obj = {b: 2}; ({a: 1, ...obj})",
+            expected: { value: { a: 1, b: 2 } },
+        },
+        {
+            code: "var obj = {b: 2}; obj = {}; ({a: 1, ...obj})",
+            expected: null,
+        },
         { code: "({ get a() {} })", expected: null },
         { code: "({ a })", expected: null },
         { code: "({ a: b })", expected: null },

test/get-string-if-constant.mjs

@@ -43,8 +43,10 @@ describe("The 'getStringIfConstant' function", () => {
         for (const { code, expected } of [
             { code: "id", expected: null },
             { code: "const id = 'abc'; id", expected: "abc" },
-            { code: "let id = 'abc'; id", expected: null },
-            { code: "var id = 'abc'; id", expected: null },
+            { code: "let id = 'abc'; id", expected: "abc" },
+            { code: "var id = 'abc'; id", expected: "abc" },
+            { code: "let id = 'abc'; id = 'foo'; id", expected: null },
+            { code: "var id = 'abc'; id = 'foo'; id", expected: null },
             { code: "const id = otherId; id", expected: null },
         ]) {
             it(`should return ${JSON.stringify(expected)} from ${code}`, () => {