Variable Differences in JavaScript

let greeting;
var greeting;
let greeting = "Hello, World";
var greeting = "Hello, World";
const greeting = "Hello, World";
  1. let greeting
  • Can declare a variable without assignment or initialization
  • let greeting;
  • Block-scoped meaning the ability to also declare a variable in any set of curly brackets. However, be aware of your scope when assigning variables in let. Your value is contingent on the scope.
let greeting = "Hello, World";   if(true) {
let greeting = "Bye"
console.log(greeting) //=> "Bye
}
console.log(greeting) //=> "Hello, World";
  • Can be reassigned but not redeclared within scope
let greeting = "Hello, World"
greeting = "Bye, World"
let greeting = "Bye, World" // => ERROR
  • Can’t be redeclared or reassigned, always pointing to the same object. That being said, if you try to assign a const variable without a value, you cannot reassign it with a value
const greeting;
greeting = "Hello, World" //=> ERROR
  • Variable is initialized during declaration so the correct way to declare and assign is:
const greeting = "Hello, World"
  • BLOCK -SCOPED like let, meaning that they can be accessed within the scope that they were initialized
  • Can declare variable without assigning value upon initialization
var greeting;
  • Can be redeclared AND reassigned
  • function and globally scoped, meaning that we can we can redeclare and reassign within the same scope. YIKES! How convenient but you can also anticipate issues further down the road.
var greeting = "Hello, World"
var greeting = "Bye, World"
  • let and const are block scoped variables
  • const cannot be redeclared or reassigned after initialized at declaration
  • var is function and globally scoped, that can be redeclared and reassigned, which can cause scope issues! Avoid var

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Grace Yuiko Nakano

Grace Yuiko Nakano

Fullstack Software Engineer | Musician | Food Lover | Coffee Addict

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