add MissingInteger

Author: talgat-ruby

README.md

@@ -19,6 +19,7 @@
         -   [PermCheck](#s_4_perm_check)
         -   [FrogRiverOne](#s_4_frog_river_one)
         -   [MaxCounters](#s_4_max_counters)
+        -   [MissingInteger](#s_4_max_counters)
 -   [Licence](#licence)
 
 <a name="installation"></a>
@@ -169,6 +170,18 @@ https://github.com/talgat-ruby/codility-lessons-javascript/blob/master/lesson4_c
 
 https://app.codility.com/demo/results/training5T6ZHR-8ZK/
 
+<a name="s_4_missing_integer"></a>
+
+#### MissingInteger
+
+##### File
+
+https://github.com/talgat-ruby/codility-lessons-javascript/blob/master/lesson4_counting_elements/MissingInteger.js
+
+##### Link to Report
+
+https://app.codility.com/demo/results/training86F8QY-JHX/
+
 <a name="licence"></a>
 
 ## License

lesson4_counting_elements/MissingInteger.js

@@ -0,0 +1,11 @@
+function solution(A) {
+    const set = new Set(A);
+
+    let i = 1;
+    while (set.has(i)) {
+        i++;
+    }
+    return i;
+}
+
+module.exports = solution;

lesson4_counting_elements/MissingInteger.test.js

@@ -0,0 +1,13 @@
+const solution = require("./MissingInteger");
+
+describe("test  MissingInteger", () => {
+    test("[1, 3, 6, 4, 1, 2] -> 5", () => {
+        expect(solution([1, 3, 6, 4, 1, 2])).toBe(5);
+    });
+    test("[1, 2, 3] -> 4", () => {
+        expect(solution([1, 2, 3])).toBe(4);
+    });
+    test("[−1, −3] -> 1", () => {
+        expect(solution([-1, -3])).toBe(1);
+    });
+});