added array.unique method

lib/array.js

@@ -1,11 +1,24 @@
 'use strict';
 
+const checkArray = val => {
+    if (!Array.isArray(val)) {
+        throw Error('value must be an array');
+    }
+};
+
 module.exports = {
     toArray: arraylike => [...arraylike],
 
     ensureArray: val => Array.isArray(val) ? val : [val],
 
+    unique (ar) {
+        checkArray(ar);
+        return ar.filter((val, i, input) => i === input.indexOf(val));
+    },
+
     flatten: function flatten (ar) {
+        checkArray(ar);
+
         return ar.reduce((acc, item) => {
             if (Array.isArray(item)) {
                 acc.push(...flatten(item));

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@mightyplow/jslib",
-  "version": "0.4.0",
+  "version": "0.5.0",
   "description": "js, helpers library",
   "main": "jslib.js",
   "devDependencies": {