const [,value] 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: javascript const

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