Hoisting with let and const in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is hoisting in JavaScript?

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their containing scope before code execution.

intermediate

How does hoisting behave differently with let and const compared to var?

let and const are hoisted but not initialized. They stay in a 'temporal dead zone' until their declaration line, causing a ReferenceError if accessed earlier. var is hoisted and initialized with undefined.

beginner

What happens if you try to access a let variable before its declaration?

You get a ReferenceError because the variable is in the temporal dead zone until its declaration is reached.

beginner

Can you reassign a const variable after declaration?

No, const variables cannot be reassigned after they are initialized. Trying to do so causes a TypeError.

intermediate

Explain the 'temporal dead zone' in simple terms.

The temporal dead zone is the time between entering a scope and when a let or const variable is declared. During this time, the variable exists but cannot be accessed.

What will happen if you run this code?<br>

console.log(x);<br>let x = 5;

A ReferenceError

B undefined

C 5

D TypeError

Accessing let variable x before declaration causes a ReferenceError due to the temporal dead zone.

Which statement is true about const variables?

A <code>const</code> variables can be reassigned anytime.

B <code>const</code> variables are hoisted but not initialized, causing a temporal dead zone.

C <code>const</code> variables behave exactly like <code>var</code>.

D <code>const</code> variables are hoisted and initialized with undefined.

const variables are hoisted but not initialized, so accessing them before declaration causes a ReferenceError.

What is the temporal dead zone?

A The time when variables are accessible before declaration.

B A special block where variables are permanently undefined.

C The time after a variable is declared and initialized.

D The time between entering a scope and the declaration of <code>let</code> or <code>const</code> variables.

The temporal dead zone is the period after entering a scope but before let or const variables are declared.

What will this code output?<br>

var a = 1;<br>console.log(a);<br>var a = 2;<br>console.log(a);

A 1 then 2

B undefined then 2

C ReferenceError

D 2 then 2

var variables are hoisted and initialized with undefined, but here a is assigned 1 before first log, so outputs 1 then 2.

Which keyword allows hoisting with initialization to undefined?

A let

B const

C var

D function

var declarations are hoisted and initialized with undefined, unlike let and const.

Describe how hoisting works differently for var, let, and const variables.

Think about when variables become usable in the code.

Explain the concept of the temporal dead zone and why it matters when using let and const.

Consider what happens if you try to use a variable too early.