Add The Comma Operator as a javascript til

javascript/the-comma-operator.md

@@ -0,0 +1,18 @@
+# The Comma Operator
+
+I was surprised to see the result of this line of code:
+
+```javascript
+const what = (1,2,3,4);
+console.log(what); //=> 4
+```
+
+I asked around on
+[twitter](https://twitter.com/jbrancha/status/1084936559323951105) and
+learned that the syntax construct at play here is the [comma
+operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator).
+
+> The comma operator evaluates each of its operands (from left to right) and
+> returns the value of the last operand.
+
+And that is why `what` gets bound to `4`.