Add objectFromEntries(), objectMap()

Author: codethief

src/objectFromEntries.ts

@@ -0,0 +1,11 @@
+/**
+ * Type-safe version of Object.fromEntries(). Like the latter, it creates an
+ * object from the provided list of key-value pairs.
+ */
+export const objectFromEntries = <
+  const KVPairs extends ReadonlyArray<readonly [PropertyKey, unknown]>,
+>(
+  keyValuePairs: KVPairs
+): { [K in KVPairs[number] as K[0]]: K[1] } => {
+  return Object.fromEntries(keyValuePairs) as { [K in KVPairs[number] as K[0]]: K[1] };
+};

src/objectMap.ts

@@ -0,0 +1,32 @@
+import { objectFromEntries } from "./objectFromEntries";
+
+/**
+ * Union type representing the type of an arbitrary key-value pair in a given
+ * object type.
+ */
+type KeyValueType<TObj> = {
+  [K in keyof TObj]: { key: K; value: TObj[K] };
+}[keyof TObj];
+
+/**
+ * Like Array.prototype.map() but for objects, i.e. applies `mapper` to every
+ * key-value pair and builds a new object from the returned values.
+ *
+ * @param obj Object to be mapped
+ * @param keys The keys that should be mapped. We need a precise list, because
+ * functions like Object.keys(obj) or Object.entries(keys) might return
+ * additional keys not included in TObject/TKey.
+ * @param mapper Callback function which maps each (key, value) pair to a new
+ * value.
+ * @returns An object with the same keys as `obj` but with the values given by
+ * `mapper()`. The return type should be equivalent to `{ [K in TKey]: TNewValue }`
+ * but for some reason TSC doesn't think so. (Bug?)
+ */
+export const objectMap = <TObj extends unknown, TKey extends keyof TObj, TNewValue extends unknown>(
+  obj: Readonly<TObj>,
+  keys: TKey[],
+  mapper: (keyValuePair: KeyValueType<Pick<TObj, TKey>>) => TNewValue
+): { [K in readonly [TKey, TNewValue] as K[0]]: K[1] } => {
+  const keyValuePairs = keys.map((key) => [key, mapper({ key, value: obj[key] })] as const);
+  return objectFromEntries(keyValuePairs);
+};