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.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();difference between var, let, const
// var is a function scope ***
if(true){
var varVariable = 'This is var';
var varVariable = 'This is var again';
}
console.log(varVariable); // This is var again
// let is a block scope ***
if(true){
let letVariable = 'This is let';
let letVariable = 'This is let again';
// let variable can't re-define but we can re-assign value
console.log(letVariable); // let letVariable = 'This is let again';^SyntaxError: Identifier 'letVariable' has already been declared
}
console.log(letVariable); //ReferenceError: letVariable is not defined
// const variable can't re-define and re-assign value
// const is a block scope ***
if(true){
const constVarible = {
name: 'JavaScript',
age: '25 years',
};
constVarible.name = 'JS';
console.log(constVarible) // {name: 'JS',age: '25 years'} <= we can update const variable declared object
}difference between var and let
Scoping rules
/*The main difference is scoping rules.
Variables declared by var keyword are scoped to the
immediate function body (hence the function scope)
while let variables are scoped to the immediate
enclosing block denoted by { } (hence the block scope).**/
function run() {
var foo = "Foo"; //Function scope
let bar = "Bar";
console.log(foo, bar); // Foo Bar
{
var moo = "Mooo"
let baz = "Bazz"; //Block scope
console.log(moo, baz); // Mooo Bazz
}
console.log(moo); // Mooo
console.log(baz); // ReferenceError
}
run();
// Check out the source link below from stack overflow for more insightsCopyright © 2021 Codeinu
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