[TS] Support ArrayLike in batch function and loadMany (#214)

* Use ReadonlyArray instead of Array on input args

The runtime code does not alter the Arrays in question, and it is good form to prefer `ReadonlyArray<T>` over `Array<T>` (aka `T[]`) in the input position when one can.

* Use ReadonlyArray in promise result

* Support ArrayLike

Author: leebyron

src/index.d.ts

@@ -37,7 +37,7 @@ declare class DataLoader<K, V, C = K> {
    *     ]);
    *
    */
-  loadMany(keys: K[]): Promise<V[]>;
+  loadMany(keys: ArrayLike<K>): Promise<V[]>;
 
   /**
    * Clears the value at `key` from the cache, if it exists. Returns itself for
@@ -70,7 +70,8 @@ declare namespace DataLoader {
 
   // A Function, which when given an Array of keys, returns a Promise of an Array
   // of values or Errors.
-  export type BatchLoadFn<K, V> = (keys: K[]) => PromiseLike<Array<V | Error>>;
+  export type BatchLoadFn<K, V> =
+    (keys: ArrayLike<K>) => PromiseLike<ArrayLike<V | Error>>;
 
   // Optionally turn off batching or caching or provide a cache key function or a
   // custom cache instance.

src/index.js

@@ -129,13 +129,18 @@ class DataLoader<K, V, C = K> {
    *
    */
   loadMany(keys: $ReadOnlyArray<K>): Promise<Array<V>> {
-    if (!Array.isArray(keys)) {
+    if (!isArrayLike(keys)) {
       throw new TypeError(
         'The loader.loadMany() function must be called with Array<key> ' +
-        `but got: ${keys}.`
+        `but got: ${(keys: any)}.`
       );
     }
-    return Promise.all(keys.map(key => this.load(key)));
+    // Support ArrayLike by using only minimal property access
+    const loadPromises = [];
+    for (let i = 0; i < keys.length; i++) {
+      loadPromises.push(this.load(keys[i]));
+    }
+    return Promise.all(loadPromises);
   }
 
   /**
@@ -266,7 +271,7 @@ function dispatchQueueBatch<K, V>(
   batchPromise.then(values => {
 
     // Assert the expected resolution from batchLoadFn.
-    if (!Array.isArray(values)) {
+    if (!isArrayLike(values)) {
       throw new TypeError(
         'DataLoader must be constructed with a function which accepts ' +
         'Array<key> and returns Promise<Array<value>>, but the function did ' +
@@ -345,4 +350,15 @@ type LoaderQueue<K, V> = Array<{
   reject: (error: Error) => void;
 }>;
 
+// Private
+function isArrayLike(x: mixed): boolean {
+  return (
+    typeof x === 'object' &&
+    x !== null &&
+    typeof x.length === 'number' &&
+    (x.length === 0 ||
+      (x.length > 0 && Object.prototype.hasOwnProperty.call(x, x.length - 1)))
+  );
+}
+
 module.exports = DataLoader;