Store TypeScript void results as undefined values

usvm-ts/src/main/kotlin/org/usvm/machine/TsContext.kt

@@ -30,6 +30,7 @@ import org.jacodb.ets.model.EtsUndefinedType
 import org.jacodb.ets.model.EtsUnionType
 import org.jacodb.ets.model.EtsUnknownType
 import org.jacodb.ets.model.EtsValue
+import org.jacodb.ets.model.EtsVoidType
 import org.usvm.UAddressSort
 import org.usvm.UBoolExpr
 import org.usvm.UBoolSort
@@ -167,6 +168,7 @@ class TsContext(
         is EtsStringType -> addressSort
         is EtsNullType -> addressSort
         is EtsUndefinedType -> addressSort
+        is EtsVoidType -> addressSort
         is EtsUnionType -> unresolvedSort
         is EtsIntersectionType -> {
             val memberSorts = type.types.map(::typeToSort)

usvm-ts/src/test/kotlin/org/usvm/samples/lang/VoidAssignment.kt

@@ -0,0 +1,23 @@
+package org.usvm.samples.lang
+
+import org.jacodb.ets.model.EtsScene
+import org.junit.jupiter.api.Test
+import org.usvm.api.TsTestValue
+import org.usvm.util.TsMethodTestRunner
+import org.usvm.util.eq
+import org.usvm.test.util.checkers.eq as exactly
+
+class VoidAssignment : TsMethodTestRunner() {
+    override val scene: EtsScene = loadScene("/samples/lang/VoidAssignment.ts")
+
+    @Test
+    fun `assigned void call returns undefined after its mutation`() {
+        val method = getMethod("assignmentResult")
+
+        checkMatches<TsTestValue.TsNumber>(
+            method = method,
+            analysisResultNumberMatches = exactly(1),
+            { result -> result eq 1 },
+        )
+    }
+}

usvm-ts/src/test/resources/samples/lang/VoidAssignment.ts

@@ -0,0 +1,22 @@
+// @ts-nocheck
+// noinspection JSUnusedGlobalSymbols
+
+class VoidMutation {
+    value: number = 0;
+
+    change(): void {
+        this.value = 7;
+    }
+}
+
+class VoidAssignment {
+    assignmentResult(): number {
+        const instance = new VoidMutation();
+        const result = instance.change();
+        const observation = { result, value: instance.value };
+        if (observation.result === undefined && observation.value === 7) {
+            return 1;
+        }
+        return 0;
+    }
+}