Why is my variable initialized with the default value of `undefined`, if I declared it with `let` ?

Question

Hi!

Going through the Teacher's Notes, I stumbled upon the link about Hoisting, from MDN https://developer.mozilla.org/en-US/docs/Glossary/Hoisting#let_and_const_hoisting . It says:

"Variables declared with let and const are also hoisted, but unlike for var the variables are not initialized with a default value of undefined. Until the line in which they are initialized is executed, any code that accesses these variables will throw an exception."

I typed the following lines of code directly in the console. The return is commented after each line.

var a;  // returns undefined
let b;  // returns undefined
const c;  // returns Uncaught SyntaxError: Missing initializer in const declaration

My question is: Why is b initialized as undefined, if the documentation states that the JS engine should throw an exception? Did I misunderstand the documentation?

I reloaded the page and even tried the same lines of code in a new tab, but always get the same result.

Thanks so much for your help!

Answer 1 · 2021-09-14T16:42:19Z

September 14, 2021 4:42pm

It's a forward reference that throws the exception, not the declaration itself. For examples:

// example 1: both the declaration and initialization are hoisted:
console.log(a);  // will show "undefined"
var a;
// -------------------------------------------
// example 2: only the declaration is hoisted:
console.log(b);  // will cause exception
let b;

Welcome to the Treehouse Community

Looking to learn something new?

Victor Stanciu

Victor Stanciu

Why is my variable initialized with the default value of `undefined`, if I declared it with `let` ?

1 Answer

Steven Parker

Steven Parker

Victor Stanciu

Victor Stanciu