Outline
const and let were added in ES6 to solve an issue with var known as variable hoisting.
var can be reassigned to any value, but uses lexical scope. If you declare a var inside of a block, it will still exist outside of that block.
let is just like var except that it's scope is limited to the block where it was declared.
const is block-scoped just like let, but has restrictions. A const cannot be redifened or declared, but can be modified if it contains a complex data type.