Adding clean-set (#124)

* Adding clean-set

* Moving to our own cleanSet and cleanMerge.

* Oops, this can be simpler

* Allow the path to be a number

* Better typing

src/lib/clean-modify.ts

@@ -0,0 +1,62 @@
+function cleanSetOrMerge<A extends any[] | object>(
+  source: A,
+  keys: string | number | string[],
+  toSetOrMerge: any[] | object,
+  merge: boolean,
+): A {
+  const splitKeys = Array.isArray(keys) ? keys : ('' + keys).split('.');
+
+  // Going off road in terms of types, otherwise TypeScript doesn't like the access-by-index.
+  // The assumptions in this code break if the object contains things which aren't arrays or
+  // plain objects.
+  let last = copy(source) as any;
+  const newObject = last;
+
+  const lastIndex = splitKeys.length - 1;
+
+  for (const [i, key] of splitKeys.entries()) {
+    if (i !== lastIndex) {
+      // Copy everything along the path.
+      last = last[key] = copy(last[key]);
+    } else {
+      // Merge or set.
+      last[key] = merge ?
+        Object.assign(copy(last[key]), toSetOrMerge) :
+        toSetOrMerge;
+    }
+  }
+
+  return newObject;
+}
+
+function copy<A extends any[] | object>(source: A): A {
+  // Some type cheating here, as TypeScript can't infer between generic types.
+  if (Array.isArray(source)) return [...source] as any;
+  return { ...(source as any) };
+}
+
+/**
+ * @param source Object to copy from.
+ * @param keys Path to modify, eg "foo.bar.baz".
+ * @param toMerge A value to merge into the value at the path.
+ */
+export function cleanMerge<A extends any[] | object>(
+  source: A,
+  keys: string | number | string[],
+  toMerge: any[] | object,
+): A {
+  return cleanSetOrMerge(source, keys, toMerge, true);
+}
+
+/**
+ * @param source Object to copy from.
+ * @param keys Path to modify, eg "foo.bar.baz".
+ * @param newValue A value to set at the path.
+ */
+export function cleanSet<A extends any[] | object>(
+  source: A,
+  keys: string | number | string[],
+  newValue: any,
+): A {
+  return cleanSetOrMerge(source, keys, newValue, false);
+}