feat(frontend utils): created counter class

* also added tests

frontend/src/utils/counter.js

+/**
+ * Helper class just like the one in python
+ */
+class Counter {
+  /**
+   * @param {Iterable} elements
+   */
+  constructor(elements) {
+    const res = new Map();
+
+    elements.forEach(el => {
+      if (!res.has(el)) {
+        res.set(el, 0);
+      }
+      res.set(el, res.get(el) + 1);
+    });
+
+    /**
+     * @type {Map<any, number>}
+     */
+    this.res = res;
+  }
+
+  /**
+   * return the keys that have duplicates
+   * @returns {Array.<any>}
+   */
+  getDuplicatedElements() {
+    return [...this.res.entries()]
+      .filter(([, count]) => count > 1)
+      .map(([key]) => key);
+  }
+}
+
+export default Counter;

frontend/src/utils/counter.test.js

+import Counter from "./counter";
+
+test("Count nothing empty string", () => {
+  const counter = new Counter([]);
+  expect(counter.res).toMatchObject(new Map());
+});
+
+test("Count one element", () => {
+  const counter = new Counter(["coucou"]);
+  expect(counter.res).toMatchObject(new Map([["coucou", 1]]));
+});
+
+test("Count twice element", () => {
+  const counter = new Counter(["coucou", "coucou"]);
+  expect(counter.res).toMatchObject(new Map([["coucou", 2]]));
+});
+
+test("Count two elements diff", () => {
+  const counter = new Counter(["coucou", "coucou", "test"]);
+  expect(counter.res).toMatchObject(new Map([["coucou", 2], ["test", 1]]));
+});
+
+test("Test show duplicated", () => {
+  const counter = new Counter(["coucou", "coucou", "test"]);
+  expect(counter.getDuplicatedElements()).toMatchObject(["coucou"]);
+});