const in es6 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: js define constant by element id

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

Example 3: const js +=

const foo; // Errror as const cannot be kept un-initialized;
const foo = "foo";
console.log(foo); //Output: foo
foo += " bar"; //Error