Expand unpack() to be more expressive

lib/www/client/source/src/lib/unpack.js

@@ -1,4 +1,33 @@
-
+/** Unpacks attributes from array items.
+ *
+ * At it simplest, given an array of objects,
+ * the call unpack(rows, "x") returns an array
+ * of the "x" attribute of every item in rows.
+ *
+ * `key` may also be:
+ *
+ * - a function with the signature
+ *   (Object) => any
+ *   the result of applying the function to
+ *   the object will be used as the unpacked
+ *   value.
+ *
+ * - an array of strings, functions or other
+ *   arrays. In this case, it does a recursive
+ *   fold operation. NOTE: it mutates `key`.
+ *
+ */
 export default function unpack(rows, key) {
-  return rows && rows.map( row => row[key] );
+  if (typeof key === "function") {
+    return rows && rows.map( row => key(row) );
+  } else if (Array.isArray(key)) {
+    const car = key.shift();
+    if (key.length) {
+      return unpack(unpack(rows, car), key);
+    } else {
+      return unpack(rows, car);
+    }
+  } else {
+    return rows && rows.map( row => row?.[key] );
+  }
 };