feat: add js comma operator content (#1829)

content/roadmaps/106-javascript/content/108-javascript-expressions-and-operators/108-comma-operators.md

@@ -1 +1,7 @@
-# Comma operators
+# Comma operators
+
+The comma operator (`,`) evaluates each of its operands (from left to right) and returns the value of the last operand. This lets you create a compound expression in which multiple expressions are evaluated, with the compound expression's final value being the value of the rightmost of its member expressions. This is commonly used to provide multiple parameters to a `for` loop.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator'>Comma operator</BadgeLink>