Variables, Scope, and Memory

Question 1

What are primitive values?

Answer 1

Simple atomic pieces of data

Question 2

What are reference values?

Answer 2

Objects that may be made up of multiple values

Question 3

What are the 5 primitive types?

Answer 3

Undefined, Null, Boolean, Number, String

Question 4

What is an example of a reference type in JavaScript?

Answer 4

an Object. When you manipulate an object, you’re really working on a reference to that object rather than the actual object.

Question 5

Can you add properties to primitive values?

Answer 5

No. Only to reference values.

Question 6

What is the value of num2?

var num1 = 5;
var num2 = num1;
var num1 = 6;

Answer 6

5

Question 7

When a reference value is assigned to a variable what is the actual value of the variable?

Answer 7

A pointer to an object stored on the heap

Question 8

var obj1 = new Object();
var obj2 = obj1;
obj1.name = "Scott"

What is the value of obj2.name?

Answer 8

“Scott”

Question 9

What does passed by value mean?

Answer 9

a value is copied from one variable to another.

Question 10

What does passed by reference mean?

Answer 10

the location of the value in the memory is passed from one variable to another (Objects)

Question 11

How are function arguments passed in ECMAScript?

Answer 11

They are passed by value. Function arguments should be considered nothing more than local variables within functions.

Question 12

What is the instanceof operator used for and what does the syntax look like?

Answer 12

it is used to determine what type of object an object is.

result = variable instanceof constructor

Question 13

What does this return?

person instanceof Object

Answer 13

true (if person is an object.)

Question 14

typeof(obj) => true

If you want more information about this object what operator can you use?

Answer 14

obj instanceof Object => true

obj instanceof Array => false

Question 15

What is the global execution context?

Answer 15

The outermost context. It changes depending on the host environment but in web browsers the global context is the window object.

Question 16

What is a variable object?

Answer 16

Each execution context has an associated variable context upon which all of its defined variables and functions exist.

Question 17

What is the purpose of a scope chain?

Answer 17

to provide ordered access to all variables and functions that an execution context has access to.

Question 18

What is the front of the scope chain?

Answer 18

the variable object of the context whose code is executing. If the context is a function, then the activation object is used as the variable object.

Question 19

What does an activation object start with?

Answer 19

a single defined variable called arguments. (this doesn’t exist in global context)

Question 20

Do function arguments follow the same access rules as other variables in the execution context?

Answer 20

Yes, function arguments are considered to be variables.

Question 21

What are the two primary types of executions contexts?

Answer 21

global and function

Question 22

which two statements can cause a temporary addition to the front of the scope chain that is later removed after code execution?

Answer 22

1. The catch block in a try-catch statement

2. A with statement

Question 23

Are there block-level scopes in JavaScript?

Answer 23

No.

Question 24

When a variable is declared using var, what context is it added to?

Answer 24

The most immediate context

Question 25

If a variable is initiated without first being declared what context does it get added to?

Answer 25

The global context

Question 26

Is it faster to access local variables or global variables?

Answer 26

Local variables because there’s no need to search up the scope chain.

Question 27

What is a garbage collection language?

Answer 27

The execution environment is responsible for managing the memory required during code execution.

Question 28

What is mark-and-sweep?

Answer 28

The most popular form of garbage collection. When a variable goes in context it’s flagged as in context. When a variable goes out of context it’s flagged as out of context.

When the garbage collector runs it marks all variables stored in memory, then it clears the marks off of variables that are in context and variables referenced by in context variables. The garbage collector then does a memory sweep destroying each of the marked values and reclaiming the memory.

Question 29

What is reference counting?

Answer 29

A type of garbage collection. The idea is that every value keeps track of how many references are made to it. When the count goes to zero its safe to reclaim the associated memory.

Question 30

What is a circular reference?

Answer 30

When object A has a pointer to object B and object B has a reference to object A.

Question 31

Why should you break the connection between native javascript objects and DOM elements?

myObject.element = null;
element.someObject = null;

Answer 31

To avoid circular reference problems.

Question 32

Where are primitive values stored?

Answer 32

They are a fixed size so they are stored in memory on the stack

Question 33

Where are reference values stored?

Answer 33

Reference values are objects and are stored in memory on the heap.

Question 34

What should you do with global variables to help with memory reclamation?

Answer 34

They should be dereferenced when they are no longer needed.