Add Collection.find with function

src/util/Collection.js

@@ -77,20 +77,32 @@ class Collection extends Map {
   }
 
   /**
-   * Returns a single item where `item[key] === value`
+   * Returns a single item where `item[key] === value`, or the given function returns `true`. In the latter case,
-   * @param {string} key The key to filter by
+   * this is identical to
-   * @param {*} value The expected value
+   * [Array.find()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find).
+   * @param {string|function} keyOrFn The key to filter by, or the function to test with
+   * @param {*} [value] The expected value - required if using a key for the first param
    * @returns {*}
    * @example
    * collection.find('id', '123123...');
+   * @example
+   * collection.find(val => val.id === '123123...');
    */
-  find(key, value) {
+  find(keyOrFn, value) {
-    if (typeof key !== 'string') throw new TypeError('Key must be a string.');
+    if (typeof keyOrFn === 'string') {
-    if (typeof value === 'undefined') throw new Error('Value must be specified.');
+      if (typeof value === 'undefined') throw new Error('Value must be specified.');
-    for (const item of this.values()) {
+      for (const item of this.values()) {
-      if (item[key] === value) return item;
+        if (item[keyOrFn] === value) return item;
+      }
+      return null;
+    } else if (typeof keyOrFn === 'function') {
+      for (const [key, val] of this) {
+        if (keyOrFn(val, key, this)) return val;
+      }
+      return null;
+    } else {
+      throw new Error('First parameter must be a string key or a function.');
     }
-    return null;
   }
 
   /**