Merge pull request #26828 from Microsoft/narrowUnknownByObject

Narrowing unknown by typeof object to object | null

Author: RyanCavanaugh

src/compiler/checker.ts

@@ -14965,6 +14965,9 @@ namespace ts {
                     return type;
                 }
                 if (assumeTrue && !(type.flags & TypeFlags.Union)) {
+                    if (type.flags & TypeFlags.Unknown && literal.text === "object") {
+                        return getUnionType([nonPrimitiveType, nullType]);
+                    }
                     // We narrow a non-union type to an exact primitive type if the non-union type
                     // is a supertype of that primitive type. For example, type 'any' can be narrowed
                     // to one of the primitive types.

tests/baselines/reference/narrowUnknownByTypeofObject.js

@@ -0,0 +1,14 @@
+//// [narrowUnknownByTypeofObject.ts]
+function foo(x: unknown) {
+    if (typeof x === "object") {
+        x
+    }
+}
+
+
+//// [narrowUnknownByTypeofObject.js]
+function foo(x) {
+    if (typeof x === "object") {
+        x;
+    }
+}

tests/baselines/reference/narrowUnknownByTypeofObject.symbols

@@ -0,0 +1,13 @@
+=== tests/cases/compiler/narrowUnknownByTypeofObject.ts ===
+function foo(x: unknown) {
+>foo : Symbol(foo, Decl(narrowUnknownByTypeofObject.ts, 0, 0))
+>x : Symbol(x, Decl(narrowUnknownByTypeofObject.ts, 0, 13))
+
+    if (typeof x === "object") {
+>x : Symbol(x, Decl(narrowUnknownByTypeofObject.ts, 0, 13))
+
+        x
+>x : Symbol(x, Decl(narrowUnknownByTypeofObject.ts, 0, 13))
+    }
+}
+

tests/baselines/reference/narrowUnknownByTypeofObject.types

@@ -0,0 +1,16 @@
+=== tests/cases/compiler/narrowUnknownByTypeofObject.ts ===
+function foo(x: unknown) {
+>foo : (x: unknown) => void
+>x : unknown
+
+    if (typeof x === "object") {
+>typeof x === "object" : boolean
+>typeof x : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>x : unknown
+>"object" : "object"
+
+        x
+>x : object | null
+    }
+}
+

tests/cases/compiler/narrowUnknownByTypeofObject.ts

@@ -0,0 +1,6 @@
+// @strictNullChecks: true
+function foo(x: unknown) {
+    if (typeof x === "object") {
+        x
+    }
+}