var vs let vs const
var:
- hoisted (always declared at top of scope, global if none)
- function scope
let:
- block scope
- not redeclarable
const:
- block scope
- not reassignable
- not redeclarable
Note: Although it may seem like these hold only semantic meaning, using the
appropriate keywords helps the JS engines' compiler to decide on what to optimize.difference between let, var and const in java script
//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
*/let and var difference
function run() {
var foo = "Foo";
let bar = "Bar";
console.log(foo, bar);
{
let baz = "Bazz";
console.log(baz);
}
console.log(baz); // ReferenceError
}
run();Copyright © 2021 Codeinu
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