JS Fundamentals

Question 1

What is the difference between undefined, null and undeclared?

Answer 1

Undefined is a value that has been declared but not defined.

Null is an intentional value to convey the absence of a value.

Undeclared is value that has not been declared (using var, let, const, etc.).

Question 2

What is a primitive value?

Answer 2

Data that is not an object and has no methods and are immutable.

Question 3

What are the six primitive values?

Answer 3

1. ) String
2. ) Number
3. ) BigInt
4. ) Boolean
5. ) Undefined
6. ) Symbol

Question 4

Of what type are variables that are NOT primitive?

Answer 4

Object

Question 5

What are the two basic groups of data types in JavaScript?

Answer 5

Primitive and Reference Types

Question 6

What is hoisting?

Answer 6

JavaScript’s default behavior of moving variable declarations, function declarations and class declarations to the top.

Question 7

What’s the difference between declarations and initializations?

Answer 7

Declaration: declaring a variable (ex. var text;)

Initialization: initializing a value for a variable (text = “123”)

Question 8

What gets hoisted to the top?

Answer 8

Declarations NOT initializations!! Function declarations and NOT function expressions!

Question 9

What value is a variable assigned when declared with “var”?

Answer 9

undefined

Question 10

What value is a variable assigned when declared with “const” or “let”?

Answer 10

It will not be initialized. It will result in an error

Question 11

Are JavaScript classes hoisted?

Answer 11

Yes, only class declarations. They result in same as variables declared with “const” or “let” where they are uninitialized until evaluation. Safer to write a class and then use it.

Class expressions are NOT hoisted!

Question 12

Can an arrow function be used as a constructor function?

Answer 12

No. Arrow functions do not have its own “new.target” property so it cannot be used as a constructor function.

It cannot be called by “new” as there is no internal method that allows it.

They also don’t have the “prototype” property.

Question 13

What is “new.target”?

Answer 13

A property that lets you detect whether a function or constructor was called using the “new” operator.

In constructors and functions invoked using the new operator, “new.target” returns a reference to the constructor or function.

In normal function calls, “new.target” is undefined.

Question 14

What are the types of scope available in JavaScript?

Answer 14

There are two types:

1. ) Local Scope
2. ) Global Scope

Question 15

What type of scope does JavaScript have?

Answer 15

Function scope, where each function creates a new scope.

Question 16

What are local variables in JavaScript?

Answer 16

Variables that are only visible in the function where it was defined.

They are created when a function starts and deleted when the function is completed.

Question 17

What are global variables in JavaScript?

Answer 17

Variable declared outside a function. Can be accessed by any function.

Question 18

What happens when you assign a value to a variable that hasn’t been declared?

Answer 18

It becomes global.

Question 19

What happens when you assign a value to a variable that hasn’t been declared, but you’re running in “strict mode”?

Answer 19

The variable does NOT become global.

Question 20

What is the call method?

Answer 20

A method that allows you to use one method on different objects.

Question 21

What is the apply method?

Answer 21

Like the “call” method, a method that allows you to use one method on different objects.

Question 22

What’s the difference between the .call and .apply methods?

Answer 22

They are the same except that the “call” method accepts each parameter to be passed individually whereas the “apply” method accepts an array of parameters as the second parameter. The first parameter is the context of “this”.

Question 23

How do you call the “call” method?

Answer 23

{method you want to be shared}.call({context of “this”}, {parameters, delimited by a comma})

Question 24

How do you call the “apply” method?

Answer 24

{method you want to be shared}.apply({context of “this”}, [{array of parameters}])

Question 25

What happens when you don’t pass all the arguments needed for a method where you use “call” or “apply”?

Answer 25

The rest of the parameters will be undefined.

Question 26

What does the “bind” method do?

Answer 26

It creates a new function of the function that it’s being called on and attaches the “this” context to it so that it can be called at a later time.

Question 27

What is a closure?

Answer 27

A function that can access the variables of an outer scope.

It’s important to note that an outer function will not have access to an inner scopes’ variables.

Question 28

Why would we use a closure?

Answer 28

1. ) Data Privacy
2. ) Currying
3. ) Event Handlers
4. ) Async Callback Functions

Question 29

Is JavaScript a single or multi-threaded language?

Answer 29

Single

Question 30

What is a call stack?

Answer 30

A stack that receives line by line of code by the interpreter. As they execute, they pop from the stack (from the back).

Question 31

What is the event table?

Answer 31

When the interpreter, while in the process of placing code lines into the call stack, comes across asynchronous code, it places it in the event table.

A table that holds asynchronous code and moves it to an event queue once the allotted time necessary for the code to run happens.

Question 32

What is the event queue?

Answer 32

A queue that receives asynchronous code from the event table and is responsible for passing it to the event loop.

Question 33

What is the event loop?

Answer 33

Process that continuously checks on the call stack and the event queue.

Makes sure to take code from the event queue (from the front) once the call stack is empty.

Question 34

What is one benefit to using “let” vs “var” in async code and closures?

Answer 34

“let” creates a new binding for variables, while “var” keeps the same binding.