From dc0b091c31046823ea5ee78bdd606183621f7436 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Tue, 10 Mar 2020 08:46:20 +0100
Subject: [PATCH] LibJS: Implement basic boolean coercion

We can't expect the conditionals in "if" and "while" statements to
always return a bool, so we need to know how to boolify a JS::Value.
---
 Libraries/LibJS/AST.cpp   |  9 ++++-----
 Libraries/LibJS/Value.cpp | 19 +++++++++++++++++++
 Libraries/LibJS/Value.h   |  1 +
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index 00ed1227bf..e1260c1804 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -70,7 +70,7 @@ Value IfStatement::execute(Interpreter& interpreter) const
 {
     auto predicate_result = m_predicate->execute(interpreter);
 
-    if (predicate_result.as_bool())
+    if (predicate_result.to_boolean())
         return interpreter.run(*m_consequent);
     else
         return interpreter.run(*m_alternate);
@@ -79,7 +79,7 @@ Value IfStatement::execute(Interpreter& interpreter) const
 Value WhileStatement::execute(Interpreter& interpreter) const
 {
     Value last_value = js_undefined();
-    while (m_predicate->execute(interpreter).as_bool()) {
+    while (m_predicate->execute(interpreter).to_boolean()) {
         last_value = interpreter.run(*m_body);
     }
 
@@ -213,9 +213,8 @@ Value BinaryExpression::execute(Interpreter& interpreter) const
 
 Value LogicalExpression::execute(Interpreter& interpreter) const
 {
-    auto lhs_result = m_lhs->execute(interpreter).as_bool();
-
-    auto rhs_result = m_rhs->execute(interpreter).as_bool();
+    auto lhs_result = m_lhs->execute(interpreter).to_boolean();
+    auto rhs_result = m_rhs->execute(interpreter).to_boolean();
     switch (m_op) {
     case LogicalOp::And:
         return Value(lhs_result && rhs_result);
diff --git a/Libraries/LibJS/Value.cpp b/Libraries/LibJS/Value.cpp
index 54e3185dff..30ce077737 100644
--- a/Libraries/LibJS/Value.cpp
+++ b/Libraries/LibJS/Value.cpp
@@ -53,6 +53,25 @@ String Value::to_string() const
     ASSERT_NOT_REACHED();
 }
 
+bool Value::to_boolean() const
+{
+    switch (m_type) {
+    case Type::Boolean:
+        return m_value.as_bool;
+    case Type::Number:
+        return m_value.as_double == 0 || m_value.as_double == -0;
+    case Type::Null:
+    case Type::Undefined:
+        return false;
+    case Type::String:
+        return String(as_string()).is_empty();
+    case Type::Object:
+        return true;
+    default:
+        ASSERT_NOT_REACHED();
+    }
+}
+
 const LogStream& operator<<(const LogStream& stream, const Value& value)
 {
     return stream << value.to_string();
diff --git a/Libraries/LibJS/Value.h b/Libraries/LibJS/Value.h
index ed00df7d89..ecf0ccc4a6 100644
--- a/Libraries/LibJS/Value.h
+++ b/Libraries/LibJS/Value.h
@@ -113,6 +113,7 @@ public:
     }
 
     String to_string() const;
+    bool to_boolean() const;
 
 private:
     Type m_type { Type::Undefined };