Voice of a Developer: JavaScript Data Types

Sumit Jolly
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JavaScript is a language of the Web. This series of articles will talk about my observations learned during my decade of software development experience with JavaScript and what mistakes generally developers make and what differences they should be aware of.

Data types

A very common word and ry vebasic in programming language, JavaScript provides different data types to hold different types of values.

Q: How many data types are there in JS?

Ans: There are two types of data types in JavaScript.

Primitive data type

String
Number
Boolean
Undefined
Null

Non-primitive (reference) data type

Object
Array
RegExp

Q: How do you specify data type in JS?

Ans: It’s simple and could be defined using var keyword only

Ex-

var x=10; // holds number
var x=”javascript”; // holds string

As a developer, I am always curious to know the size of datatypes. But there is no sizeof function available which provides me size in JavaScript. But it’s good to know memory consumption when we work on bigger applications.

Download sizeof.js

Download one of the files below and upload it to your web server.

Example-

// define an object
var object =
{
'boolean' : true,
'number' : 1,
'string' : 'a',
'array' : [1, 2, 3]
};
// determine the size of the object
var size = sizeof(object);
//output: size is 92 bytes

Q: What is the difference in Undefined and Null?

Ans: In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such a,

var TestVar;
alert(TestVar); //shows undefined
alert(typeof TestVar); //shows undefined
null is an assignment value. It can be assigned to a variable as no value:
var TestVar = null;
alert(TestVar); //shows null
alert(typeof TestVar); //shows object

Objects

Objects are variables too. However, objects can contain many values. Ex-

var person = {age:59, address:"Delhi”};

Q: Do you know simple way to create objects?

Ans:

var obj={};
//undefined
typeof(obj);
"object"
Object Properties
You can access object properties in two ways:
objectName.propertyName
or
objectName["propertyName"]

Ex-

person.age
Or
Person[“age”]

Object Methods

You access an object method with the following syntax:

objectName.methodName()

Example

var person = {age:59, address:"Delhi", fullName:function(){console.log('John Doe');return 'John Doe';}};

If you access the property without parenthesis (), it will return the function definition:

person.fullName

// function () {console.log('John Doe'); return 'John Doe';}
Otherwise it’ll execute the method
person.fullName();
// John Doe

Loop over all properties in Object

We often use FOR statement to loop, but there is another feature, for in, which is pretty useful to iterate over all properties of Object

Ex-

for(a in person){console.log(a)}
age
address
fullName

Q: How do you add/delete properties to existing object?

Ans: It is simple in Javascript, by giving a value

Ex-

person.country = “India”;

To delete you could use delete keyword

Ex-

delete person.age;

Access operator

Goto browser console window and try to access all properties /methods via access operators, ex-

Q: Can you access a property, which do not exist in Object?

Ans: Yes, you can access a property on an object that does not exist but will result in undefined.

Ex-

person.lastName;
//undefined

Q: Can you access a method, which do not exist in Object?

Ans: No, you cannot access a method on an object that does not exist because it’ll yield an error.

Ex-

person.lastName();