Variables

Question 1

What is a variable

Answer 1

A variable is simply a named area of a program’s memory space where the program can store data.

Typically, variables can be changed. That is, we can give the variable a new value.

Question 2

What are variable names often referred to by the broader term?

Answer 2

Identifiers

Question 3

In JavaScript, what do identifiers refer to?

Answer 3

Variable names declared by let and var

Constant names declared by const

Property names of objects (Global Objects)

Function names

Function parameters

Class names

Question 4

What else is a variable?

Answer 4

Variables declared with let and var

Constants declared with const

Properties of the global object

Function names

Function parameters

Class names

Question 5

Functions and classes are variable names?

Answer 5

Functions and classes are actually values in JavaScript, and their names are used in the same way as more traditional variables.

Question 6

What is a variable declaration?

Answer 6

It is a statement that asks the JavaScript engine to reserve space for a variable with a particular name.

Optionally, it also specifies an initial value for the variable: undefined

Question 7

What can we see from this?

Answer 7

From this we can see that the variable ‘firstName’ was initialized with a value of ‘undefined’

Question 8

How can we assign a more useful value?

Answer 8

We do so by using an INITIALIZER in the declaration:

As you can see, we’ve now stored the string ‘Mitchell’ in memory. We can use firstName as a label for that string elsewhere in the program

Question 9

Is ‘firstName’ permanently assigned to ‘Mitchell’?

Answer 9

NO! In fact, we can reassign it to any other value, not just strings:

Question 10

What is the return value of an assignment?

Answer 10

It is the value on the right-hand side of the = operator

Question 11

What do declarations return?

Answer 11

Declarations don’t actually return a value at all.

This is because declarations are statements. Statements don’t return values.

Note that regardless of whether we provide a value in a declaration, the variable is initialized. If we don’t provide an explicit value, that initial value is ‘undefined’

Question 12

What are assignments and reassignments?

Answer 12

Expressions

Question 13

Differences in terminology surrounding the ‘=’ operator

Answer 13

When used in a declaration, the = is just a syntactic token that tells JavaScript that you’re going to supply an initial value for the variable.

However, in an assignment, the = is called the assignment operator. For example, in let firstName = ‘Mitchell’, the = is a syntactic token, but in firstName = ‘Martha’, it is the assignment operator.

Question 14

Do variables have values that are deeply linked to each other?

Answer 14

You’ll notice that b retains the value 4, even though a is now 7. This example suggests that variables have values that aren’t deeply-linked to each other. If you change one variable, it doesn’t change other variables with the same value.

Question 15

‘const’ keyword?

Answer 15

The ‘const’ keyword is similar to ‘let’, but it lets you declare and initialize constant identifiers

Question 16

Are constants immutable?

Answer 16

Constants have an immutable binding to their values. Unlike an ordinary variable, once you declare a constant, you cannot assign it a new value. The constant will continue to have that value until the constant is no longer needed.

Using constants is a great way to label a value with a name that makes your code more descriptive and easier to understand

Question 17

What is the standard convention when naming constants?

Answer 17

It is to use all uppercase letters and digits in the name; if the name contains multiple words, use underscores to separate the words.

Question 18

Do ‘const’ declarations require a value?

Answer 18

Yes!

Question 19

What is ‘variable scope’?

Answer 19

Determines where it is available in the program

The location where you declare a variable determines its scope

Question 20

What is ‘block scope’?

Answer 20

A block is a related set of JavaScript statements and expressions between a pair of opening and closing curly braces.

The term block usually refers to executable code between braces, including function bodies

Question 21

Is everything between curly braces technically a block?

Answer 21

No!

For instance, the braces that surround an object literal do not define a block.

Technically, the braces that surround a function body don’t define a block either, though it is convenient to think of function bodies as blocks.

While there are similarities between blocks, function bodies, and, to a lesser degree, object literals, the term block usually refers to executable code between braces, including function bodies

Question 22

In general, where do blocks appear?

Answer 22

Blocks appear in if…else, while, do…while, for, switch, and try…catch statements, or by themselves (as in the first example above)

Question 23

Why does the console print ‘bar’?

Answer 23

This code prints the string “bar” since a is accessible inside the block. Thus, we can reassign it to a different value inside the block

Variables declared with ‘const’ have the same scope as ‘let’ variables.

Question 24

Does the third type of variable declaration ‘var’ use block scoping?

Answer 24

NO!

Question 25

In JavaScript, can you create a variable by merely assigning a value to it without declaring it variable or constant?

Answer 25

YES!

That looks harmless, but JavaScript has some bizarre rules when working with undeclared variables. The most notable rule is that all undeclared variables have global scope: they ignore block and function scope entirely. If your program uses that same variable name in a different scope without declaring it, there’s a good chance that it will step on the original variable by changing its content. You don’t want that to happen: it’s typically difficult to debug, and sometimes fixing it breaks other code.