added new problems.

src/EventEmitter.js

@@ -151,8 +151,8 @@ const emitor = new Emitter();
 const callback1 = function(x, y) {console.log(x + y)}
 const callback2 = function(x, y) {console.log(y + x)}
 
-// emitor.addListener('event_1', callback1);
-// emitor.addListener('event_1', callback2);
+emitor.addListener('event_1', callback1);
+emitor.addListener('event_1', callback2);
 //
 // console.log('emitor', emitor);
 //

src/chainableArthmatic.js

@@ -0,0 +1,25 @@
+const init = num => {
+  return new ChainableArtimatic(num);
+};
+
+class ChainableArtimatic {
+  constructor(num) {
+    this._value = num;
+  }
+
+  value() {
+    return this._value;
+  }
+
+  add(num) {
+    this._value = this._value + num;
+    return this;
+  }
+}
+
+console.log(
+  init(3)
+    .add(4)
+    .add(2)
+    .value()
+);

src/flattenArr.js

@@ -0,0 +1,16 @@
+// input [1, 3, [4,6], 0, [ [3, 9], 88]]
+// output [1, 3, 4, 6, 0, 3, 9, 88]
+
+const flattenArr = (arr, flatArr = []) => {
+  arr.forEach(el => {
+    if (Array.isArray(el)) {
+      return flattenArr(el, flatArr);
+    }
+    flatArr.push(el);
+  });
+
+  return flatArr;
+};
+
+const test1 = [1, 3, [4, 6], 0, [[3, 9], 88]];
+console.log(flattenArr(test1));

src/isPalindrome.js

@@ -0,0 +1,25 @@
+// O(2N) => O(N)
+const isPalindrome = str => {
+  let isValidPalindrome = true;
+  let stack = [];
+
+  Array.from(str).forEach(char => {
+    stack.push(char);
+  });
+
+  Array.from(str).forEach(char => {
+    const poppedChar = stack.pop();
+
+    if (char !== poppedChar) {
+      isValidPalindrome = false;
+    }
+  });
+
+  return isValidPalindrome;
+};
+
+const testStrings = ["abba", "tacocat", "hannah", "cat", "dog"];
+
+testStrings.forEach(str => {
+  console.log(isPalindrome(str));
+});

src/playground.js

@@ -0,0 +1 @@
+console.log('[]==0', []===0);