ES6 : var vs. let and const

scoping.js

// BAD - the dogYear variable leaks out to the window scope
var age = 100;
  if(age > 12) {
    var dogYears = age * 7;
    console.log(`You are ${dogYears} dog years old!`);
  }
  console.log(dogYears); // 700 

// GOOD - the dogYears variable stays available only to the block
var age = 100;
  if(age > 12) {
    let dogYears = age * 7;
    console.log(`You are ${dogYears} dog years old!`);
  }
  console.log(dogYears); // dogYears is NOT DEFINED bc let is available only to the block