diff --git a/chapter02/package.json b/chapter02/package.json
index 859480d..894b0a7 100644
--- a/chapter02/package.json
+++ b/chapter02/package.json
@@ -15,10 +15,7 @@
     "author": "Federico Kereki",
     "license": "ISC",
     "babel": {
-        "presets": [
-            "env",
-            "flow"
-        ]
+        "presets": ["env", "flow"]
     },
     "eslintConfig": {
         "parserOptions": {
@@ -31,16 +28,12 @@
             "node": true,
             "es6": true
         },
-        "extends": [
-            "eslint:recommended",
-            "plugin:flowtype/recommended"
-        ],
-        "plugins": [
-            "babel",
-            "flowtype"
-        ],
+        "extends": ["eslint:recommended", "plugin:flowtype/recommended"],
+        "plugins": ["babel", "flowtype"],
         "rules": {
-            "no-console": "off"
+            "no-console": "off",
+            "no-var": "error",
+            "prefer-const": "error"
         }
     },
     "prettier": {
diff --git a/chapter02/src/arrow_functions.js b/chapter02/src/arrow_functions.js
new file mode 100644
index 0000000..67abf76
--- /dev/null
+++ b/chapter02/src/arrow_functions.js
@@ -0,0 +1,40 @@
+/* @flow */
+/* eslint-disable no-unused-vars, one-var */
+
+function Show(value: mixed): void {
+    this.saved = value;
+    setTimeout(function() {
+        console.log(this.saved);
+    }, 1000);
+}
+
+const w = new Show("Doesn't work..."); // instead, "undefined" is shown
+
+function Show1(value: mixed): void {
+    this.saved = value;
+    setTimeout(
+        function() {
+            console.log(this.saved);
+        }.bind(this),
+        1000
+    );
+}
+
+function Show2(value: mixed): void {
+    this.saved = value;
+    const that = this;
+    setTimeout(function() {
+        console.log(that.saved);
+    }, 2000);
+}
+
+function Show3(value: mixed): void {
+    this.saved = value;
+    setTimeout(() => {
+        console.log(this.saved);
+    }, 3000);
+}
+
+const x = new Show1("This");
+const y = new Show2("always");
+const z = new Show3("works");
diff --git a/chapter02/src/opaque_types.js b/chapter02/src/opaque_types.js
new file mode 100644
index 0000000..b16e1fb
--- /dev/null
+++ b/chapter02/src/opaque_types.js
@@ -0,0 +1,40 @@
+/* @flow */
+/* eslint-disable no-unused-vars */
+
+opaque type dniType: string = string;
+type nameType = string; // not opaque!
+
+const stringToDni = (st: string): dniType => {
+    /*
+        do validations on st
+        if OK, return a dniType
+        if wrong, throw an error
+    */
+    return (st: dniType);
+};
+
+export type { dniType, nameType };
+export { stringToDni };
+
+class Client {
+    id: number;
+    dni: dniType;
+    name: nameType;
+    securityToken: string;
+
+    constructor(anId: number, aDni: dniType, aName: nameType) {
+        this.id = anId;
+        this.dni = aDni;
+        this.name = aName;
+
+        this.securityToken = "generate.some.token.somehow";
+    }
+
+    showNameAndDni(t: string): void {
+        console.log(`${t} - Name: ${this.name} DNI: ${this.dni}`);
+    }
+
+    useTokenForSomething(): void {
+        // do something with the token
+    }
+}
diff --git a/chapter02/src/opaque_usage.js b/chapter02/src/opaque_usage.js
new file mode 100644
index 0000000..cfac997
--- /dev/null
+++ b/chapter02/src/opaque_usage.js
@@ -0,0 +1,28 @@
+/* @flow */
+/* eslint-disable no-unused-vars */
+
+import type { dniType, nameType } from "./opaque_types";
+import { stringToDni } from "./opaque_types";
+
+function updateClient(id: number, dni: dniType, name: nameType) {
+    /*
+        Talk to some server
+        Update the dni and name for the client with given id
+    */
+}
+
+const newDni = "1234567-8"; // supposedly a DNI
+const newName = "Kari Nordmann";
+
+updateClient(229, newName, newDni); // doesn't work; 2nd argument should be a dni
+updateClient(229, newDni, newName); // doesn't work either; same reason
+updateClient(229, stringToDni(newDni), newName); // OK!
+
+/*
+    Constraints
+*/
+function showText(st: string) {
+    console.log(`Important message: ${st}`);
+}
+const anotherDni: dniType = stringToDni("9876543-2");
+showText(anotherDni); // error, if no subtype constraint is added!
diff --git a/chapter02/src/spread_and_rest.js b/chapter02/src/spread_and_rest.js
index 9046119..3ecf86e 100644
--- a/chapter02/src/spread_and_rest.js
+++ b/chapter02/src/spread_and_rest.js
@@ -1,5 +1,5 @@
 /* @flow */
-/* eslint-disable no-unused-vars */
+/* eslint-disable prefer-const,no-unused-vars,no-unused-labels */
 
 let values = [22, 9, 60, 12, 4, 56];
 const maxOfValues = Math.max(...values); // 60
@@ -14,11 +14,17 @@ let person = { name: "Juan", age: 24 };
 let copyOfPerson = { ...person };
 let expandedPerson = { ...person, sister: "María" };
 
-const average = (...nums: Array<number>): number => {
+function average(...nums: Array<number>): number {
     let sum = 0;
     for (let i = 0; i < nums.length; i++) {
         sum += nums[i];
     }
     return sum / nums.length;
-};
+}
 console.log(average(22, 9, 60, 12, 4, 56)); // 27.166667
+
+const simpleAction = (t: string, d: mixed): Object => {
+    type: t;
+    data: d;
+    return {};
+};
diff --git a/chapter02/src/types_basic.js b/chapter02/src/types_basic.js
index 8cc533c..7f023e3 100644
--- a/chapter02/src/types_basic.js
+++ b/chapter02/src/types_basic.js
@@ -24,10 +24,17 @@ numbersList[1] = "SEP"; // error; cannot assign a string to a number
 
 let anotherList: number[];
 
-let sealedObject: { name: string, age?: number };
+let sealedObject: { name: string, age?: number } = { name: "" };
 sealedObject.name = "Ivan Horvat"; // OK
 sealedObject.id = 229; // error: key isn't defined in the data type
-sealedObject = { age: 57 }; // error: mandatory field is missing
+sealedObject = { age: 57 }; // error: mandatory "name" field is missing
 
 let unsealedObject = {};
 unsealedObject.id = 229; // OK
+
+const toString2 = (x: number): string => {
+    return x + "x";
+};
+
+type numberToString = number => string;
+const toString3: numberToString = (x: number) => String(x);