fix(51521): Using a of property declared after an initializing constructor triggers an assertion failure in JS

src/compiler/utilities.ts

@@ -3531,7 +3531,7 @@ export function isSpecialPropertyDeclaration(expr: PropertyAccessExpression | El
 export function setValueDeclaration(symbol: Symbol, node: Declaration): void {
     const { valueDeclaration } = symbol;
     if (!valueDeclaration ||
-        !(node.flags & NodeFlags.Ambient && !(valueDeclaration.flags & NodeFlags.Ambient)) &&
+        !(node.flags & NodeFlags.Ambient && !isInJSFile(node) && !(valueDeclaration.flags & NodeFlags.Ambient)) &&
         (isAssignmentDeclaration(valueDeclaration) && !isAssignmentDeclaration(node)) ||
         (valueDeclaration.kind !== node.kind && isEffectiveModuleDeclaration(valueDeclaration))) {
         // other kinds of value declarations take precedence over modules and assignment declarations

tests/baselines/reference/ambientPropertyDeclarationInJs.errors.txt

@@ -0,0 +1,27 @@
+/test.js(3,9): error TS2322: Type '{}' is not assignable to type 'string'.
+/test.js(6,5): error TS8009: The 'declare' modifier can only be used in TypeScript files.
+/test.js(6,19): error TS8010: Type annotations can only be used in TypeScript files.
+/test.js(9,19): error TS2339: Property 'foo' does not exist on type 'string'.
+
+
+==== /test.js (4 errors) ====
+    class Foo {
+        constructor() {
+            this.prop = {};
+            ~~~~~~~~~
+!!! error TS2322: Type '{}' is not assignable to type 'string'.
+        }
+
+        declare prop: string;
+        ~~~~~~~
+!!! error TS8009: The 'declare' modifier can only be used in TypeScript files.
+                      ~~~~~~
+!!! error TS8010: Type annotations can only be used in TypeScript files.
+
+        method() {
+            this.prop.foo
+                      ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'string'.
+        }
+    }
+

tests/baselines/reference/ambientPropertyDeclarationInJs.symbols

@@ -0,0 +1,24 @@
+=== /test.js ===
+class Foo {
+>Foo : Symbol(Foo, Decl(test.js, 0, 0))
+
+    constructor() {
+        this.prop = {};
+>this.prop : Symbol(Foo.prop, Decl(test.js, 1, 19), Decl(test.js, 3, 5))
+>this : Symbol(Foo, Decl(test.js, 0, 0))
+>prop : Symbol(Foo.prop, Decl(test.js, 1, 19), Decl(test.js, 3, 5))
+    }
+
+    declare prop: string;
+>prop : Symbol(Foo.prop, Decl(test.js, 1, 19), Decl(test.js, 3, 5))
+
+    method() {
+>method : Symbol(Foo.method, Decl(test.js, 5, 25))
+
+        this.prop.foo
+>this.prop : Symbol(Foo.prop, Decl(test.js, 1, 19), Decl(test.js, 3, 5))
+>this : Symbol(Foo, Decl(test.js, 0, 0))
+>prop : Symbol(Foo.prop, Decl(test.js, 1, 19), Decl(test.js, 3, 5))
+    }
+}
+

tests/baselines/reference/ambientPropertyDeclarationInJs.types

@@ -0,0 +1,28 @@
+=== /test.js ===
+class Foo {
+>Foo : Foo
+
+    constructor() {
+        this.prop = {};
+>this.prop = {} : {}
+>this.prop : string
+>this : this
+>prop : string
+>{} : {}
+    }
+
+    declare prop: string;
+>prop : string
+
+    method() {
+>method : () => void
+
+        this.prop.foo
+>this.prop.foo : any
+>this.prop : string
+>this : this
+>prop : string
+>foo : any
+    }
+}
+

tests/baselines/reference/quickInfoAtPropWithAmbientDeclarationInJs.baseline

@@ -0,0 +1,38 @@
+=== /a.js ===
+// class C {
+//     constructor() {
+//         this.prop = "";
+//     }
+//     declare prop: string;
+//     method() {
+//         this.prop.foo
+//                   ^^^
+// | ----------------------------------------------------------------------
+// | any
+// | ----------------------------------------------------------------------
+//     }
+// }
+
+[
+  {
+    "marker": {
+      "fileName": "/a.js",
+      "position": 122,
+      "name": ""
+    },
+    "item": {
+      "kind": "",
+      "kindModifiers": "",
+      "textSpan": {
+        "start": 119,
+        "length": 3
+      },
+      "displayParts": [
+        {
+          "text": "any",
+          "kind": "keyword"
+        }
+      ]
+    }
+  }
+]

tests/cases/compiler/ambientPropertyDeclarationInJs.ts

@@ -0,0 +1,16 @@
+// @allowJs: true
+// @checkJs: true
+// @noEmit: true
+// @filename: /test.js
+
+class Foo {
+    constructor() {
+        this.prop = {};
+    }
+
+    declare prop: string;
+
+    method() {
+        this.prop.foo
+    }
+}

tests/cases/fourslash/quickInfoAtPropWithAmbientDeclarationInJs.ts

@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowJs: true
+// @filename: /a.js
+////class C {
+////    constructor() {
+////        this.prop = "";
+////    }
+////    declare prop: string;
+////    method() {
+////        this.prop.foo/**/
+////    }
+////}
+
+verify.baselineQuickInfo();