Variable is the fundamental part in programming. We declare variable to store different data types. To declare a variable we use the key word _var_, _let_ and _const_. A variable can declared at different scope. In this section we will see the scope, scope of variables when we use var or let.
Variables scopes can be:
- Window
- Global
- Local
Variable can be declared globally or locally or window scope. We will see both global and local scope.
Anything declared without let, var or const is scoped at window level.
Let us image we have a scope.js file.
### Window Scope
Without using console.log() open your browser and check, you will see the value of a and b if you write a or b on the browser. That means a and b are already available in the window.
```js
//scope.js
a = 'JavaScript' // is a window scope this found anywhere
b = 10 // this is a window scope variable
function letsLearnScope() {
console.log(a, b)
if (true) {
console.log(a, b)
}
}
console.log(a, b) // accessible
```
### Global scope
A globally declared variable can be access every where in the same file. But the term global is relative. It can be global to the file or it can be global relative to some block of codes.
```js
//scope.js
let a = 'JavaScript' // is a global scope it will be found anywhere in this file
let b = 10 // is a global scope it will be found anywhere in this file
function letsLearnScope() {
console.log(a, b) // JavaScript 10, accessible
if (true) {
let a = 'Python'
let b = 100
console.log(a, b) // Python 100
}
console.log(a, b)
}
letsLearnScope()
console.log(a, b) // JavaScript 10, accessible
```
### Local scope
A local declared variable can be access only certain block code.
```js
//scope.js
let a = 'JavaScript' // is a global scope it will be found anywhere in this file
let b = 10 // is a global scope it will be found anywhere in this file
function letsLearnScope() {
console.log(a, b) // JavaScript 10, accessible
let c = 30
if (true) {
// we can access from the function and outside the function but
// variables declared inside the if will not be accessed outside the if block
let a = 'Python'
let b = 20
let d = 40
console.log(a, b, c) // Python 20 30
}
// we can not access c because c's scope is only the if block
console.log(a, b)
}
letsLearnScope()
console.log(a, b) // JavaScript 10, accessible
```
Now, you have an understanding of scope. A variable declared with *var* only scoped to function but variable declared with *let* or *const* is block scope(function block, if block, loop etc)
```js
//scope.js
function letsLearnScope() {
var gravity = 9.81
console.log(gravity)
}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined
if (true){
var gravity = 9.81
console.log(gravity) // 9.81
}
console.log(gravity) // 9.81
for(var i = 0; i <3;i++){
console.log(i) // 1, 2, 3
}
console.log(i)
```
In ES6 and above there is *let* and *const*, so you will suffer from the sneakiness of *var*. When we use *let* our variable is block scope and it will not infect other parts of our code.
```js
//scope.js
function letsLearnScope() {
// you can use let or const, but gravity is constant I prefer to use const
const gravity = 9.81
console.log(gravity)
}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined
if (true){
const gravity = 9.81
console.log(gravity) // 9.81
}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined
for(let i = 0; i <3;i++){
console.log(i) // 1, 2, 3
}
// console.log(i), Uncaught ReferenceError: gravity is not defined
The scope *let* and *const* is the same. The difference is only reassigning. We can not change or reassign the value of const variable. I would strongly suggest you to use *let* and *const*, by using *let* and *const* you will writ clean code and avoid hard to debug mistakes. As a rule of thumb, you can use *let* for any value which change, *const* for any constant value, array, object, arrow function and function expression.
Everything can be an object and objects do have properties and properties have values, so an object is key value pair. The order of the key is not reserved, or there is no order.
Creating an object literal. To create an object literal, we use two curly brackets.
### Creating an empty object
An empty object
```js
const person = {}
```
### Creating an objecting with values
Now, the person object has firstName, lastName, age, location, skills and isMarried properties. The value of properties or keys could be a string, number, boolean, an object, null, undefined or a function.
Let us see some examples of object. Each key has a value in object.
```js
const rectangle = {
length: 20,
width: 20
}
console.log(rectangle) // {length: 20, width: 20}
const person = {
firstName: 'Asabeneh',
lastName: 'Yetayeh',
age: 250,
country: 'Finland',
city: 'Helsinki',
skills: [
'HTML',
'CSS',
'JavaScript',
'React',
'Node',
'MongoDB',
'Python',
'D3.js'
],
isMarried: true
}
console.log(person)
```
### Getting values from an object
We can access values of object using two methods:
- using . followed by key name if the key-name is a one word
- using square bracket and a quote
```js
const person = {
firstName: 'Asabeneh',
lastName: 'Yetayeh',
age: 250,
country: 'Finland',
city: 'Helsinki',
skills: [
'HTML',
'CSS',
'JavaScript',
'React',
'Node',
'MongoDB',
'Python',
'D3.js'
],
getFullName: function() {
return `${this.firstName}${this.lastName}`
},
'phone number': '+3584545454545'
}
// accessing values using .
console.log(person.firstName)
console.log(person.lastName)
console.log(person.age)
console.log(person.location)
// value can be accessed using square bracket and key name
console.log(person['firstName'])
console.log(person['lastName'])
console.log(person['age'])
console.log(person['age'])
console.log(person['location'])
// for instance to access the phone number we only use the square bracket method
console.log(person['phone number'])
```
### Creating object methods
Now, the person object has getFullName properties. The getFullName is function inside the person object and we call it an object method. The _this_ key word refers to the object itself. We can use the word _this_ to access the values of different properties of the object. We can not use an arrow function as object method because the word this refers to the window inside an arrow function instead of the object itself. Example of object:
```js
const person = {
firstName: 'Asabeneh',
lastName: 'Yetayeh',
age: 250,
country: 'Finland',
city: 'Helsinki',
skills: [
'HTML',
'CSS',
'JavaScript',
'React',
'Node',
'MongoDB',
'Python',
'D3.js'
],
getFullName: function() {
return `${this.firstName} ${this.lastName}`
}
}
console.log(person.getFullName())
// Asabeneh Yetayeh
```
### Setting new key for an object
An object is a mutable data structure and we can modify the content of an object after it gets created.
Setting a new keys in an object
```js
const person = {
firstName: 'Asabeneh',
lastName: 'Yetayeh',
age: 250,
country: 'Finland',
city: 'Helsinki',
skills: [
'HTML',
'CSS',
'JavaScript',
'React',
'Node',
'MongoDB',
'Python',
'D3.js'
],
getFullName: function() {
return `${this.firstName} ${this.lastName}`
}
}
person.nationality = 'Ethiopian'
person.country = 'Finland'
person.title = 'teacher'
person.skills.push('Meteor')
person.skills.push('SasS')
person.isMarried = true
person.getPersonInfo = function() {
let skillsWithoutLastSkill = this.skills
.splice(0, this.skills.length - 1)
.join(', ')
let lastSkill = this.skills.splice(this.skills.length - 1)[0]
let skills = `${skillsWithoutLastSkill}, and ${lastSkill}`
let fullName = this.getFullName()
let statement = `${fullName} is a ${this.title}.\nHe lives in ${this.country}.\nHe teaches ${skills}.`
return statement
}
console.log(person)
console.log(person.getPersonInfo())
```
```sh
Asabeneh Yetayeh is a teacher.
He lives in Finland.
He teaches HTML, CSS, JavaScript, React, Node, MongoDB, Python, D3.js, Meteor, and SasS.
```
### Object Methods
There are different methods to manipulate an object. Let us see some of the available methods.
_Object.assign_: To copy an object without modifying the original object
🌕 You are astonishing. Now, you are super charged with the power of objects. You have just completed day 7 challenges and you are 7 steps a head in to your way to greatness. Now do some exercises for your brain and for your muscle.
1. Add name, legs, color, age and bark properties for the dog object. The bark property is a method which return _woof woof_
1. Get name, legs, color, age and bark value from the dog object
1. Set new properties the dog object: breed, getDogInfo
1. Create an object literal called _personAccount_. It has _firstName, lastName, incomes, expenses_ properties and it has _totalIncome, totalExpense, accountInfo,addIncome, addExpense_ and _accountBalance_ methods. Incomes is a set of incomes and its description and expenses is a set of incomes and its description.
1. Count logged in users,count users having greater than equal to 50 points from the following object.
{country: 'United States of America', population: 323947000}
]```
````
1. \*\*\* Try to develop a program which calculate measure of central tendency of a sample(mean, median, mode) and measure of variability(range, variance, standard deviation). In addition to those measures find the min, max, count, percentile, and frequency distribution of the sample. You can create an object called statistics and create all the functions which do statistical calculations as method for the statistics object. Check the output below.
