Add swapCase

snippets/swapCase.md

@@ -0,0 +1,22 @@
+---
+title: swapCase
+tags: string,beginner
+---
+
+Creates a string with uppercase characters converted to lowercase and vice versa.
+
+- Use the spread operator (`...`) to convert `str` into an array of characters.
+- Use `String.prototype.toLowerCase()` and `String.prototype.toUpperCase()` to convert lowercase characters to uppercase and vice versa.
+- Use `Array.prototype.map()` to apply the transformation to each character, `Array.prototype.join()` to combine back into a string.
+- Note that it is not necessarily true that `swapCase(swapCase(str)) === str`.
+
+```js
+const swapCase = str =>
+  [...str]
+    .map(c => (c === c.toLowerCase() ? c.toUpperCase() : c.toLowerCase()))
+    .join('');
+```
+
+```js
+swapCase('Hello world!'); // 'hELLO WORLD!'
+```