New Variables — Creation, Updating and Scoping

Question 1

What is the most important new feature of Let and Const variables?

Answer 1

They are Block Scoped.

var is function scoped.

Question 2

Can var variables be updated?

Answer 2

Yes

Question 3

Can var variables be redefined?

Answer 3

Yes

Question 4

Name the most important benefit of using let and consts and explain why it is important.

Answer 4

Rather than being scoped to the function, it is scoped to the block.

This prevents variables leaking out of blocks into parent functions and even the window in certain cases.

Question 5

How do you spot a block statement?

Name an example.

Answer 5

An opening and closing bracket is a block statement.

e.g an if statement

if {
}

Question 6

Can you update a variable

Answer 6

Yes

Question 7

Can you redefine a redefine a variable in the same scope

Answer 7

Yes

Question 8

Can you redefine a let in the same scope

Answer 8

No, it will throw an error

Question 9

Why is it useful that let and const don’t allow you to redefine them?

Answer 9

Prevents duplicates through accidents that can cause bugs.

Question 10

Can const variables be updated?

Answer 10

No. But with objects properties my be updated.

Question 11

Can let variables be updated?

Answer 11

Yes

Question 12

In a script tag you write:

let result = true inside an if statement;

what would you get if you logged result in the browser and why?

Answer 12

Nothing because the let variable is block scoped locally inside an if statement.

Question 13

In a script tag you write:

let result = true

what would you get if you logged result in the browser and why?

Answer 13

You would get ‘true’.

Because the let var is block scoped it would be attached to the window and can therefore be found by a console.log

Question 14

Can you reassign a const variable object.

Name an example that illustrates this relationship.

Answer 14

You can’t reassign them. The same object will always be used and cannot be wiped.

Properties updatable.

Just like a person you wouldn’t change their name or body, but their age be changed.

Question 15

Two reasons for using let and const

Answer 15

Scope something to a block

Creating a variable that cannot be changed

Question 16

What does an IIFE stand for?

Answer 16

Immediately invoked function expression

Question 17

What is an IIFE used for and why is was it useful?

Answer 17

An immediately invoked function expression is a self running function often used to prevents var variables leaking into the global scope.

Question 18

What is the ES6 answer to the IIFE?

Answer 18

let and const are block scoped and therefore just require two brackets:

{
   let name = "harry"
}

Question 19

What is a problem loops with var variables suffer from?

Answer 19

The var variable in a loop is function scoped and therefore leaks into the window or parent scope.

Just a placeholder value so this behaviour is not necessarily desired.

Question 20

Name one benefit of using let variables for loops?

Answer 20

The loop’s let variable is block scoped to the loop itself helping to reduce bugs.

Question 21

What is the temporal dead zone?

Answer 21

Cannot access variable before it is created.

When variables are accessed before they are defined.
These return undefined.

e.g. console.log(car)
< TEMPORAL DEAD ZONE >
var car = “audi”;

Question 22

How do let and const behave with the temporal dead zone?

Answer 22

They provide a proper error which will break code as opposed to var’s which just return undefined.

Question 23

Is var dead? What Should I use?

Answer 23

Var should never be used.

Use Const by default.

Use Let if you need to update it.