Javascript variable and its declaration

javascript variable types : var, let, const

These are three type of javascript variabe declaration.
var
====
Function scoped or globally Scoped.
Reassignable and Redeclarable
Not subject to temporal dead zone.

var language = "JavaScript";
function foo() {
  var language = "Python";
  framework = "React";
  console.log(language);
}
foo(); // "Python"
console.log(language); // "JavaScript"
console.log(window.language); // "JavaScript"
console.log(window.framework); // "React"


let
====
Block scoped.
Re-assignable, not re-declarable.
Subjected to temporal dead zone.

let language = "JavaScript";
{
  let language = "React"; // Block scope
  console.log(language); // "React"
}
function foo() {
  let language = "Python";
  console.log(language);
}
foo(); // "Python"
console.log(language); // "JavaScript"
console.log(window.language); // undefined

Const
====
Block scoped.
Not re-assignable, not re-declarable.
Subjected to temporal dead zone.

const language = "JavaScript";
{
  const language = "React"; // Block scope
  console.log(language); // "React"
}
function foo() {
  const language = "Python";
  console.log(language); 
}
foo(); // "Python"
console.log(language); // "JavaScript"
console.log(window.language); // undefined


All javascript variable are hoisted (Means all the variable declarations are moved to top of the scope). 
The var variable will intialise with a vlaue of undefined. But let and const type will throw ReferenceError.

eg: for let
console.log(language); // TDZ
let language = "JavaScript"; // No hoisting
// ReferenceError: language is not defined

Block scope is within curly brackets.

const something = {};
something = 10; // Error.

let somethingElse = {};
somethingElse = 1000; // This is fine.

More understanding on const type.
const myArray = [];
myArray.push(1); // Works fine.
myArray[1] = 2; // Also works fine.
console.log(myArray); // [1, 2]
myArray = [1, 2, 3] // This will throw.

const foo; // ERROR: const declarations must be initialized