Add solution file

Author: odsantos

1-js/99-js-misc/02-eval/1-eval-calculator/solution.md

@@ -0,0 +1,11 @@
+Let's use `eval` to calculate the maths expression:
+
+```js demo run
+let expr = prompt("Type an arithmetic expression?", '2*3+2');
+
+alert( eval(expr) );
+```
+
+The user can input any text or code though.
+
+To make things safe, and limit it to arithmetics only, we can check the `expr` using a [regular expression](info:regular-expressions), so that it only may contain digits and operators.