From b6307beb6ef581584603f92b2ce4af1382fff646 Mon Sep 17 00:00:00 2001
From: 0xtechnobabble <0xtechnobabble@protonmail.com>
Date: Sun, 8 Mar 2020 07:58:58 +0200
Subject: [PATCH] LibJS: Implement if statements

If statements execute a certain action or an alternative one depending
on whether the tested condition is true or false, this commit helps
establish basic control flow capabilities in the AST.
---
 Libraries/LibJS/AST.cpp | 23 +++++++++++++++++++++++
 Libraries/LibJS/AST.h   | 24 ++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index cc2004895a..49bc943ec4 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -61,6 +61,16 @@ Value ReturnStatement::execute(Interpreter& interpreter) const
     return value;
 }
 
+Value IfStatement::execute(Interpreter& interpreter) const
+{
+    auto predicate_result = m_predicate->execute(interpreter);
+
+    if (predicate_result.as_bool())
+        return interpreter.run(*m_consequent);
+    else
+        return interpreter.run(*m_alternate);
+}
+
 Value add(Value lhs, Value rhs)
 {
     ASSERT(lhs.is_number());
@@ -236,4 +246,17 @@ void ReturnStatement::dump(int indent) const
     argument().dump(indent + 1);
 }
 
+void IfStatement::dump(int indent) const
+{
+    ASTNode::dump(indent);
+
+    print_indent(indent);
+    printf("If\n");
+    predicate().dump(indent + 1);
+    consequent().dump(indent + 1);
+    print_indent(indent);
+    printf("Else\n");
+    alternate().dump(indent + 1);
+}
+
 }
diff --git a/Libraries/LibJS/AST.h b/Libraries/LibJS/AST.h
index f200f7e451..1f510e9864 100644
--- a/Libraries/LibJS/AST.h
+++ b/Libraries/LibJS/AST.h
@@ -127,6 +127,30 @@ private:
     NonnullOwnPtr<Expression> m_argument;
 };
 
+class IfStatement : public ASTNode {
+public:
+    explicit IfStatement(NonnullOwnPtr<Expression> predicate, NonnullOwnPtr<ScopeNode> consequent, NonnullOwnPtr<ScopeNode> alternate)
+        : m_predicate(move(predicate))
+        , m_consequent(move(consequent))
+        , m_alternate(move(alternate))
+    {
+    }
+
+    const Expression& predicate() const { return *m_predicate; }
+    const ScopeNode& consequent() const { return *m_consequent; }
+    const ScopeNode& alternate() const { return *m_alternate; }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "IfStatement"; }
+
+    NonnullOwnPtr<Expression> m_predicate;
+    NonnullOwnPtr<ScopeNode> m_consequent;
+    NonnullOwnPtr<ScopeNode> m_alternate;
+};
+
 enum class BinaryOp {
     Plus,
     Minus,