This content originally appeared on DEV Community and was authored by RabeDatta
Redeclaration of variable
You can redeclare a variable with var but not with let and const:
var foo = 34;
var foo = 46;
foo; // 46
But if we try to redeclare a variable with const and let, we will get a syntax error:
var foo = 34;
let foo =46;
foo;
// SyntaxError: Identifier 'foo' has already been declared.
Global scope & function scope vs block scope
Var only knows global scope and function scope meaning if you declare a var variable inside an if statement, you can still access it from outside of that if statement, but you can't do with let and const. For example:
var name = "Max";
if(name === 'Max'){
var hobbies = ['Sports', 'Cooking'];
console.log(hobbies) // [ 'Sports', 'Cooking' ]
}
console.log(name, hobbies)
// 'Max' [ 'Sports', 'Cooking' ]
Notice that you can still access hobbies from outside of that if block. But in the case of let and const, we can't do that since both let & const are block scoped.
var name = "Max";
if(name === 'Max'){
let hobbies = ['Sports', 'Cooking'];
console.log(hobbies) // [ 'Sports', 'Cooking' ]
}
console.log(name, hobbies)
// ReferenceError: hobbies is not defined
As you can see, we got a ReferenceError. Let and cost only care about curly braces(except for curly braces of an object). You can't access variable defined inside a child block from outside. Also, you can't access variable that you defined inside a nested block.
var name = "Max";
if(name){
{
let test = 34;
}
console.log(test)
// ReferenceError: test is not defined
}
Hoisting: var vs let & cost
- when it comes to var, JavaScript initializes the hoisted variable as undefined:
console.log(name); //'undefined'
var name = "Rob"
- when it comes to let & const, the declarations remain uninitialized (Cannot be Hoisted) :
console.log(name); // ReferenceError:
Cannot access 'name' before initialization
var name = "Rob"
This content originally appeared on DEV Community and was authored by RabeDatta
RabeDatta | Sciencx (2022-01-16T07:05:36+00:00) var vs let & const.. Retrieved from https://www.scien.cx/2022/01/16/var-vs-let-const/
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