From 520311eb8bd705ae793d82522f461f981bcd8922 Mon Sep 17 00:00:00 2001
From: Stephan Unverwerth <s.unverwerth@gmx.de>
Date: Fri, 3 Apr 2020 19:11:31 +0200
Subject: [PATCH] LibJS: Add short circuit logical evaluation When evaluating
 logical binop expressions, the rhs must not be evaluated if the lhs leads to
 the whole expression not being truthy.

---
 Libraries/LibJS/AST.cpp                         | 17 +++++++++++++----
 .../Tests/logical-expressions-short-circuit.js  | 15 +++++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)
 create mode 100644 Libraries/LibJS/Tests/logical-expressions-short-circuit.js

diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index 763c799170..f96b798c43 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -282,17 +282,26 @@ Value LogicalExpression::execute(Interpreter& interpreter) const
     auto lhs_result = m_lhs->execute(interpreter);
     if (interpreter.exception())
         return {};
-    auto rhs_result = m_rhs->execute(interpreter);
-    if (interpreter.exception())
-        return {};
+
     switch (m_op) {
     case LogicalOp::And:
-        if (lhs_result.to_boolean())
+        if (lhs_result.to_boolean()) {
+            auto rhs_result = m_rhs->execute(interpreter);
+            if (interpreter.exception())
+                return {};
+
             return Value(rhs_result);
+        }
+
         return Value(lhs_result);
     case LogicalOp::Or:
         if (lhs_result.to_boolean())
             return Value(lhs_result);
+
+        auto rhs_result = m_rhs->execute(interpreter);
+        if (interpreter.exception())
+            return {};
+
         return Value(rhs_result);
     }
 
diff --git a/Libraries/LibJS/Tests/logical-expressions-short-circuit.js b/Libraries/LibJS/Tests/logical-expressions-short-circuit.js
new file mode 100644
index 0000000000..105281f62f
--- /dev/null
+++ b/Libraries/LibJS/Tests/logical-expressions-short-circuit.js
@@ -0,0 +1,15 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    let foo = 1;
+    false && (foo = 2);
+    assert(foo === 1);
+
+    foo = 1;
+    true || (foo = 2);
+    assert(foo === 1);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}