LibJS: Parse comma operator into SequenceExpression

Author: https://github.com/linusg
Commit: 8ffdcce0d0
Pull-request: https://github.com/SerenityOS/serenity/pull/2193

Libraries/LibJS/Tests/comma-operator.js

@@ -0,0 +1,26 @@
+load("test-common.js");
+
+try {
+    assert((1, 2, 3) === 3);
+    assert((1, 2 + 3, 4) === 4);
+
+    var foo = 0;
+    foo = (foo++, foo);
+    assert(foo === 1);
+
+    var a, b, c;
+    assert((a = b = 3, c = 4) === 4);
+    assert(a === 3);
+    assert(b === 3);
+    assert(c === 4);
+
+    var x, y, z;
+    assert((x = (y = 5, z = 6)) === 6);
+    assert(x === 6)
+    assert(y === 5)
+    assert(z === 6)
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}