var vs let vs const,

//let is used to declare the variable or initialize a variable.. //let variable can be updated in future. //Its block scope and we cannot redeclare it. let a; // Initializing the variable. let b = 4; // Initializing and declaring variable. let b = 5; //Error:- cannot redeclare a value but we can update the value. //var is just like let but var is used to declare a variable value at top of //the program, its hoisted. var a; // Initializing the variable. var b = 4; // Initializing and declaring variable. var b = 5; //b will be redeclared and we can update its value. //const variable should be declared and intialized at same time. //const variable cannot be updated in future. //we cannot even redeclare the variable in future. const a = 10; //Initilizing and declaring a variable at a time. const b; //Error:- const should be declared and intialized. const a = 11 // Error:- cannot redeclare the variable again.

difference between var and let and const in javascript

var variable1 // declared using var
const variable2 // declared using const

myFunction1();

function myFunction1() {
  variable1 = "hello!";
  console.log(variable1);
  // "hello!"
}

myFunction2();

function myFunction2() {
  variable2 = "hello!";
  // error: variable2 is a constant and can not be redifined
}

myFunction3(true);

myFunction3(codition) {
  if(condition) {
    let variable3 = "helo!" // declared using const
  }
  variable3 = "hello!";
  // error: variable3 is out of scope
}

/*

Therefore, 
  
  var:
    -can be declared outside any function to be used inside any function
    -can be declared inside any function or any other {} which are of only if or if-else or switch etc. and can be used anywhere inside the function
    -can be changed again and again anywhere
  let:
    -can be declared outside any function to be used inside any function
    -if declared inside any function or any other {} which are of only if or if-else or switch etc. and can't be used anywhere inside the function and can be only used inside statement
    - can be changed again and again only inside the statement in which they are made in
    const:
    -can be declared outside any function to be used inside any function
    -can be declared inside any function or any other {} which are of only if or if-else or switch etc. and can be used anywhere inside the function
    -cannot be changed again and agan anywhere, if tried to, will result in an error

*/

Posted by: Guest on July-23-2021

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