Add `.find` poly

app/assets/javascripts/extensions/array.js → app/assets/javascripts/extensions/array.js.es6

@@ -6,3 +6,19 @@ Array.prototype.first = function() {
 Array.prototype.last = function() {
   return this[this.length-1];
 }
+
+Array.prototype.find = Array.prototype.find || function(predicate, ...args) {
+  if (!this) throw new TypeError('Array.prototype.find called on null or undefined');
+  if (typeof predicate !== 'function') throw new TypeError('predicate must be a function');
+
+  const list = Object(this);
+  const thisArg = args[1];
+  let value = {};
+
+  for (let i = 0; i < list.length; i += 1) {
+    value = list[i];
+    if (predicate.call(thisArg, value, i, list)) return value;
+  }
+
+  return undefined;
+};

spec/javascripts/extensions/array_spec.js → spec/javascripts/extensions/array_spec.js.es6

@@ -11,13 +11,36 @@
         return expect(arr.first()).toBe(0);
       });
     });
-    return describe('last', function() {
+    describe('last', function() {
       return it('returns the last item', function() {
         var arr;
         arr = [0, 1, 2, 3, 4, 5];
         return expect(arr.last()).toBe(5);
       });
     });
+
+    describe('find', function () {
+      beforeEach(() => {
+        this.arr = [0, 1, 2, 3, 4, 5];
+      });
+
+      it('returns the item that first passes the predicate function', () => {
+        expect(this.arr.find(item => item === 2)).toBe(2);
+      });
+
+      it('returns undefined if no items pass the predicate function', () => {
+        expect(this.arr.find(item => item === 6)).not.toBeDefined();
+      });
+
+      it('error when called on undefined or null', () => {
+        expect(Array.prototype.find.bind(undefined, item => item === 1)).toThrow();
+        expect(Array.prototype.find.bind(null, item => item === 1)).toThrow();
+      });
+
+      it('error when predicate is not a function', () => {
+        expect(Array.prototype.find.bind(this.arr, 1)).toThrow();
+      });
+    });
   });
 
 }).call(this);