Short circuit evaluation

index.js

@@ -40,12 +40,30 @@ module.exports = function (ast, vars) {
         }
         else if (node.type === 'BinaryExpression' ||
                  node.type === 'LogicalExpression') {
+            var op = node.operator;
+
+            if (op === '&&') {
+                var l = walk(node.left);
+                if (l === FAIL) return FAIL;
+                if (!l) return l;
+                var r = walk(node.right);
+                if (r === FAIL) return FAIL;
+                return r;
+            }
+            else if (op === '||') {
+                var l = walk(node.left);
+                if (l === FAIL) return FAIL;
+                if (l) return l;
+                var r = walk(node.right);
+                if (r === FAIL) return FAIL;
+                return r;
+            }
+
             var l = walk(node.left);
             if (l === FAIL) return FAIL;
             var r = walk(node.right);
             if (r === FAIL) return FAIL;
 
-            var op = node.operator;
             if (op === '==') return l == r;
             if (op === '===') return l === r;
             if (op === '!=') return l != r;
@@ -62,8 +80,6 @@ module.exports = function (ast, vars) {
             if (op === '|') return l | r;
             if (op === '&') return l & r;
             if (op === '^') return l ^ r;
-            if (op === '&&') return l && r;
-            if (op === '||') return l || r;
 
             return FAIL;
         }

test/eval.js

@@ -121,3 +121,29 @@ test('constructor at runtime only', function(t) {
     var res = evaluate(ast);
     t.equal(res, undefined);
 });
+
+test('short circuit evaluation AND', function(t) {
+    t.plan(1);
+
+    var variables = {
+        value: null
+    };
+    var src = 'value && value.func()';
+    var ast = parse(src).body[0].expression;
+    var res = evaluate(ast, variables);
+    t.equals(res, null);
+})
+
+test('short circuit evaluation OR', function(t) {
+    t.plan(1);
+
+    var fnInvoked = false;
+    var variables = {
+        value: true,
+        fn: function() { fnInvoked = true}
+    };
+    var src = 'value || fn()';
+    var ast = parse(src).body[0].expression;
+    evaluate(ast, variables);
+    t.equals(fnInvoked, false);
+})