ui: class-map helper (#12202)

{{class-map}} is used to easily add a list of classes, conditionally, and
have them all formatted nicely ready to be printed in a DOM class attribute.

For ease, as well as using entries, you can also just provide a simple string
without the boolean and that class will always be added.

ui/packages/consul-ui/app/helpers/class-map.js

@@ -0,0 +1,17 @@
+import { helper } from '@ember/component/helper';
+
+/**
+ * Conditionally maps classInfos (classes) to a string ready for typical DOM
+ * usage (i.e. space delimited)
+ *
+ * @typedef {([string, boolean] | [string])} classInfo
+ * @param {(classInfo | string)[]} entries - An array of 'entry-like' arrays of `classInfo`s to map
+ */
+const classMap = entries => {
+  const str = entries
+    .filter(entry => (typeof entry === 'string' ? true : entry[entry.length - 1]))
+    .map(entry => (typeof entry === 'string' ? entry : entry[0]))
+    .join(' ');
+  return str.length > 0 ? str : undefined;
+};
+export default helper(classMap);

ui/packages/consul-ui/app/helpers/class-map.mdx

@@ -0,0 +1,45 @@
+# class-map
+
+`{{class-map}}` is used to easily add a list of classes, conditionally, and
+have them all formatted nicely ready to be printed in a DOM `class` attribute.
+
+For ease, as well as using entries, you can also just provide a simple string
+without the boolean and that class will always be added.
+
+```hbs preview-template
+<figure>
+  <figcaption>
+    The correct classes added/omitted
+  </figcaption>
+  <div
+    class={{class-map
+      'component-name'
+      (array 'add-this-class' true)
+      (array 'dont-add-this-class' false)
+      'simple-string-class'
+    }}
+    ...attributes
+  >
+    <code>
+      class="{{class-map
+        (array 'add-this-class' true)
+        (array 'dont-add-this-class' false)
+        'simple-string-class'
+      }}"
+    </code>
+  </div>
+</figure>
+```
+
+## Positional Arguments
+
+| Argument | Type | Default | Description |
+| --- | --- | --- | --- |
+| `entries` | `(classInfo \| string)[]` |  | An array of 'entry-like' arrays of `classInfo`s to map |
+
+## Types
+
+| Type | Default | Description |
+| --- | --- | --- |
+| `classInfo` | `([string, boolean] \| [string])` |  |
+