var-let-const.md

var

• create variables that are limited to function scope

• variables once assigned can be reassigned with different values

• variables with var declaration are hoisted to the top of function definition & assigned undefined till its actual initialization

if (true) {
  console.log(age); // undefined
  var age = 25;
}
age = 26; // since var is not block scoped, can access & reassign variables from the block
console.log(age); // 26

let

• create variables that are block scoped (if-else, loops etc.)

• we can reassign the existing let variable to any value

• variables with let declaration are hoisted to the top of function definition and assigned as undefined

• However, javascript engine cannot use it till the initialization which is called temporal dead zone

• let variables in the block are garbage collected after execution of that block, so we can't access them outside the block

if (true) {
  console.log(age); // ReferenceError: Cannot access 'age' before initialization
  let age = 25;
}
console.log(age); // ReferenceError: age is not defined

const

• create constant variables which are block scoped

• const variables are needed to be initialized at the time of declaration

• cannot reassign const variables after creating them

• However objects and arrays declared with const can be mutated, i.e. values in them can be reassigned but not the entire object or array

• const variables in the block are also garbage collected after execution of that block & cannot be accessed outside the block

const age = 25;
age = 26; // cannot reassign to const variable