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.