LibJS: Improve UpdateExpression::execute()

- Let undefined variables throw a ReferenceError by using
  Identifier::execute() rather than doing variable lookup manually and
  ASSERT()ing
- Coerce value to number rather than ASSERT()ing
- Make code DRY
- Add tests

Libraries/LibJS/AST.cpp

@@ -775,11 +775,10 @@ Value UpdateExpression::execute(Interpreter& interpreter) const
 {
     ASSERT(m_argument->is_identifier());
     auto name = static_cast<const Identifier&>(*m_argument).string();
-
+    auto old_value = m_argument->execute(interpreter);
-    auto previous_variable = interpreter.get_variable(name);
+    if (interpreter.exception())
-    ASSERT(previous_variable.has_value());
+        return {};
-    auto previous_value = previous_variable.value();
+    old_value = old_value.to_number();
-    ASSERT(previous_value.is_number());
 
     int op_result = 0;
     switch (m_op) {
@@ -789,14 +788,13 @@ Value UpdateExpression::execute(Interpreter& interpreter) const
     case UpdateOp::Decrement:
         op_result = -1;
         break;
+    default:
+        ASSERT_NOT_REACHED();
     }
 
-    interpreter.set_variable(name, Value(previous_value.as_double() + op_result));
+    auto new_value = Value(old_value.as_double() + op_result);
-
+    interpreter.set_variable(name, new_value);
-    if (m_prefixed)
+    return m_prefixed ? new_value : old_value;
-        return JS::Value(previous_value.as_double() + op_result);
-
-    return previous_value;
 }
 
 void AssignmentExpression::dump(int indent) const

Libraries/LibJS/Tests/update-expressions-basic.js

@@ -0,0 +1,54 @@
+load("test-common.js");
+
+try {
+    assertThrowsError(() => {
+        ++x;
+    }, {
+        error: ReferenceError,
+        message: "'x' not known"
+    });
+
+    var n = 0;
+    assert(++n === 1);
+    assert(n === 1);
+
+    var n = 0;
+    assert(n++ === 0);
+    assert(n === 1);
+
+    var n = 0;
+    assert(--n === -1);
+    assert(n === -1);
+
+    var n = 0;
+    assert(n-- === 0);
+    assert(n === -1);
+
+    var a = [];
+    assert(a++ === 0);
+    assert(a === 1);
+
+    var b = true;
+    assert(b-- === 1);
+    assert(b === 0);
+
+    var s = "foo";
+    assert(isNaN(++s));
+    assert(isNaN(s));
+
+    var s = "foo";
+    assert(isNaN(s++));
+    assert(isNaN(s));
+
+    var s = "foo";
+    assert(isNaN(--s));
+    assert(isNaN(s));
+
+    var s = "foo";
+    assert(isNaN(s--));
+    assert(isNaN(s));
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}