const keyword instead of
// Will allow you to redefine the variable's value: var message = 'hello' message = 'goodbye' // Will throw an exception when you attempt to redefine: const message = 'hello' message = 'goodbye' // Uncaught TypeError: Assignment to constant variable.
const will raise an exception if it’s value is redefined, forcing you to create new variables instead of mutating existing ones.
Note that this will not detect changes within objects, arrays and other complex types. So this will not raise an exception:
const nums = [1, 2, 3, 4] nums.push(5) // totally valid nums =  // Will throw an exception still tho
This is because the reference to the object or array does not change. You have to actually change the reference for the
const keyword to know you’re redeclaring the value. Make sense?
And, of course you’ll need to be using something like Babel since this is a feature in ES6 (ES2015) and isn’t support in all environments/browsers yet.