what does const mean in javascript code example

Example 1: what is const in javascript

const age; // errror as const cannot be kept un-initialized;
const age = 20 ; 
const age = 21 , // error as once declared const variable cann't be
// re-declared in same scope or different scope.

Example 2: const in javascript

const value = 10;
const constant = value;

Example 3: javascript const

const b = 1; // this variable unique and can't change it.

Example 4: var or const in javascript

// var declares a variable, meaning its value will vary. 
// const declares a constant, meaning its value will remain 
// consistant and not change. 
// If your variable changes throughout the program or website, 
// declare it using a var statement. 
// Otherwise, if its value does not change, declare it using 
// a const statement. 

const myConst='A const does not change.';

var myVar='A var does change.';

var myVar=2;

Example 5: js define constant by element id

const elem = document.getElementById('elementID');

Example 6: const {} = object in javascript

/*The destructuring assignment syntax is a JavaScript expression that
makes it possible to unpack values from arrays, or properties from objects,
  into distinct variables.*/
  let array = [2,3]; 
  [a,b] = array;// unpacking array into var a and b
  console.log(a); //output 2
  console.log(b); //output 3
  let object = {name:"someone",weight:"500pounds"};
  let {name,weight} = object; // unpacking object into  var name and weight
  console.log(name);// output someone
  console.log(weight);//output 500pounds

//it i similar as doing this
/*
var a = array[0];
var b = array[1]
var name = object.name;
var weight = object.weight;

 */