feat(keys): Add module keys

Returns an array of keys of an object

src/__tests__/keys.js

@@ -0,0 +1,9 @@
+import keys, { _keys } from '../keys';
+
+test('Core.keys', () => {
+  expect(keys({ a: 1, b: 2 })).toEqual(['a', 'b']);
+});
+
+test('Internal._keys', () => {
+  expect(_keys({ a: 1, b: 2 })).toEqual(['a', 'b']);
+});

src/index.js

@@ -23,6 +23,7 @@ import isNumber from './isNumber';
 import isObject from './isObject';
 import isString from './isString';
 import join from './join';
+import keys from './keys';
 import last from './last';
 import length from './length';
 import map from './map';
@@ -64,6 +65,7 @@ export {
   isObject,
   isString,
   join,
+  keys,
   last,
   length,
   map,

src/keys.js

@@ -0,0 +1,45 @@
+/* eslint-disable no-underscore-dangle, no-restricted-syntax */
+import isObject from './isObject';
+import has from './has';
+import length from './length';
+import nth from './nth';
+
+export const _keys = obj => {
+  const hasDontEnumBug = !Object.prototype.propertyIsEnumerable.call(
+    { toString: null },
+    'toString',
+  );
+
+  const dontEnums = [
+    'toString',
+    'toLocaleString',
+    'valueOf',
+    'hasOwnProperty',
+    'isPrototypeOf',
+    'propertyIsEnumerable',
+    'constructor',
+  ];
+
+  const dontEnumsLength = length(dontEnums);
+
+  if (!isObject(obj)) throw new TypeError('Keys called on non-object');
+
+  const result = [];
+
+  for (const prop in obj) {
+    if (has(prop, obj)) result.push(prop);
+  }
+
+  if (hasDontEnumBug) {
+    for (let i = 0; i < dontEnumsLength; i++) {
+      const prop = nth(i, dontEnums);
+      if (has(prop, obj)) {
+        result.push(prop);
+      }
+    }
+  }
+
+  return result;
+};
+
+export default Object.keys || _keys;

src/map.js

@@ -3,13 +3,14 @@ import isObject from './isObject';
 import assoc from './assoc';
 import append from './append';
 import prop from './prop';
+import keys from './keys';
 
 export default (fn, arr) => {
   if (isObject(arr)) {
     return reduce(
       (acc, key) => assoc(key, fn(prop(key, arr)), acc),
       {},
-      Object.keys(arr),
+      keys(arr),
     );
   }