Variable declaration using let in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the let keyword do in JavaScript?

The let keyword declares a variable that can be changed later. It creates a block-scoped variable, meaning it only exists inside the nearest curly braces { }.

beginner

What is the difference between let and var?

let variables are block-scoped, so they only exist inside the block they are declared in. var variables are function-scoped or global-scoped, which can cause unexpected behavior.

beginner

Can you redeclare a variable using let in the same scope?

No, redeclaring a variable with let in the same scope causes an error. You can only declare it once per block.

intermediate

What happens if you try to use a let variable before it is declared?

You get a ReferenceError because let variables are in a 'temporal dead zone' until their declaration line is reached.

beginner

Example: What will this code output?<br><pre>let x = 5;<br>{<br>  let x = 10;<br>  console.log(x);<br>}<br>console.log(x);</pre>

The first console.log(x) inside the block prints 10 because it uses the inner x. The second prints 5 because it uses the outer x.

What kind of scope does a let variable have?

A Block scope

B Function scope

C Global scope only

D No scope

let variables are limited to the block they are declared in, which is called block scope.

What happens if you declare let x = 1; twice in the same block?

A It works fine, x is overwritten

B x becomes undefined

C It creates two separate variables

D SyntaxError: Identifier 'x' has already been declared

You cannot redeclare a let variable in the same block; it causes a syntax error.

Which keyword allows redeclaration of variables in the same scope?

A let

B var

C None

D const

var allows redeclaration in the same scope, unlike let and const.

What error do you get if you use a let variable before declaring it?

A ReferenceError

B TypeError

C SyntaxError

D No error

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

Which of these is true about let variables?

A They are not hoisted at all

B They are hoisted and initialized with undefined

C They are hoisted but not initialized

D They behave exactly like <code>var</code>

let variables are hoisted but not initialized, so accessing them before declaration causes an error.

Explain how let helps avoid bugs compared to var.

Think about where the variable exists and when you can use it.

Describe what happens when you declare a variable with let inside a block and outside the block with the same name.

Imagine a box inside a bigger box, each with its own label.