Add mapString, fix dig

snippets/mapString.md

@@ -0,0 +1,16 @@
+### mapString
+
+Creates a new string with the results of calling a provided function on every character in the calling string.
+
+Use `String.split('')` and `Array.map()` to call the provided function, `fn`, for each character in `str`.
+Use `Array.join('')` to recombine the array of characters into a string.
+The callback function, `fn`, takes three arguments (the current character, the index of the current character and the string `mapString` was called upon).
+
+```js
+const mapString = (str, fn) =>
+	str.split('').map((c, i) => fn(c, i, str)).join('');
+```
+
+```js
+mapString('lorem ipsum', c => c.toUpperCase()); // 'LOREM IPSUM'
+```