Hoisting:
Hoisting is a mechanism where variables and function declarations are moved to the top of their scope before code execution.
console.log(x); // prints undefined
var x = 100;
console.log(x); //prints 100
- Variables declared using var are hoisted to the top of their scope and initialized with a value of undefined(special type).
- Variables declared using let are hoisted to the top of their scope but are not initialized with any value.
- Variables declared using const are hoisted to the top of their scope but are not initialized with any value.
console.log(x); // prints undefined
var x = 100;
console.log(x); //prints 100
console.log(y); //Reference error
let y = 200;
console.log(y); //prints 200
console.log(z); //Reference error
const z = 300;
console.log(z); //prints 300
What are Datatypes?
Data types are a crucial notion in programming.
It is necessary to have some knowledge about the type in order to act on variables.
For example, 2 and 3 are numbers, and when added together, they equal 5.
JavaScript is a dynamic, loosely typed language. Variables in JavaScript are not tied to any specific value type, and each variable can be assigned (and reassigned) values of any kind.
In Javascript, there are 7 primitive datatypes:
- Boolean
- Number
- String
- Null
- Undefined
- Bigint
- Symbol
Boolean
Boolean represents a logical entity and can have two values: true and false.
var a = true //Variable 'a' holds value of boolean type.
Number
The number type represents both integer and floating point numbers (decimals and exponentials).
A variable of number type can range between [-(253 − 1),(253 − 1)].
In addition to representing floating-point numbers, the number type has three symbolic values: +Infinity, -Infinity, and NaN (“Not a Number”).
var a = 2.3 //Variable 'a' holds value of number type.
var b = 2 //Variable 'b' holds value of number type.
String
String type is used to represent textual data. A string in JavaScript must be surrounded by quotes(double, single or backticks). It is a set of elements and each element in the string occupies a position.
var a = "Hello"; //Variable 'a' holds value of string type
Null
A variable that has been declared but not been assigned a value has the value undefined.
var a; //Variable 'a' holds undefined
var b = undefined; //Variable 'a' holds undefined
BigInt
BigInt type has been recently added to the language to represent
integers of arbitrary length (numbers which cannot be represented in number type.
A BigInt value is created by appending n to the end of an integer. For e.g, 23n is a value of bigint type.
var a = 9324012456789012613321123n;
Symbol
A Symbol is a unique and immutable primitive value and may be used as the key of an Object property. It is used to create unique identifiers for objects.
More of Datatypes
Object:
Object refers to a data structure containing data and instructions for working with the data. Objects sometimes refer to real-world things, for e.g, a vehicle etc.
Objects can contain many values in form of key-value pairs.
JavaScript objects are written with curly braces {}.
var a = { //Variable 'a' stores an object which contains several key-value pairs.
name: "Prathameshj",
age: 20
}
Function
A function is a code snippet that can be called by other code or by itself, or a variable that refers to the function.
var a = function () { //Variable 'a' holds a function
console.log("Hello world!");
};
typeof Keyword
typeof is an operator which can easily help us to find the type of a certain variable.
For e.g console.log(typeof 30) prints number as output, signifying that the value 30 belongs to number type.
Type Conversion
JavaScript variables can be converted to a new variable and another data type by the use of a JavaScript function.
var a = true;
var b = String(true);
console.log(typeof a); //prints boolean
console.log(typeof b); //prints string