To understand the concept of Temporal Dead Zone (TDZ), let's look at this familiar ES5 code:
console.log(language);
var language = 'JavaScript';
This will log undefined.
In ES5, all variable declarations are processed before any code is executed.
Known as hoisting, all var declarations appear to be moved to the top. In this example, the variable language
is hoisted and declared, however; it has not been assigned the value of JavaScript yet.
Now let's look at the same code using ES6 syntax:
console.log(language);
let language = 'JavaScript';
This will throw a ReferenceError.
A common mistake is to assume that the let/const were not hoisted. The short answer is that let/const declarations are hoisted, but they will throw errors when you try to access them before a value is initialized.
How will this affect your code? If you had if-statements checking if the variable is currently set to undefined, this will require your attention and a revision of the app's logic.
For a deep dive on why let/const are different, I encourage you to read Fabrício S. Matté's post on TDZ.