Intermediate JavaScript

Question 1

What do we mean when we say a function has side effects?

Answer 1

We mean that, aside from the return value of a function, a function causes the writing of a state that extends beyond the execution period of the function or an IO event.

Question 2

Do “pure functions” have side effects?

Answer 2

No, “pure functions” do not have side effects.

Question 3

What are some examples of side effects in JavaScript functions?

Answer 3

1. Mutation of a local static variable
2. Mutation of a non-local variable
3. Mutation of a mutable reference argument
4. Production or Mutation of input/output streams

Question 4

Why are “pure functions” considered good?

Answer 4

They tend to be more reusable and more clear in their effect.

Question 5

Why might side effects be considered “tricky”

Answer 5

Side effects make the function harder to reason about (less clear) and less easy to reuse.

Question 6

Why is the consideration of side effects important when using JavaScript functions

Answer 6

Side effects should be considered because some functions are useful because of their side effect, while others may cause unintended actions.

Question 7

Name a JavaScript function primarily used for its side effects

Answer 7

The forEach array method is primarily used for its side effects

Question 8

What is referential transparency?

Answer 8

It means that you can replace a function call with the resulting value and not change the meaning of the program

Question 9

What is function composition?

Answer 9

Function composition is the idea of combing functions to produce a new function or deeper computation. Think of the way React is compositional in nature; using components.

Question 10

What system or construct does Javascript use to handle asynchronous function calls?

Answer 10

It uses the event loop to handle asynchronous function calls.

Question 11

How is the “call stack” used in relation to running Javascript code?

Answer 11

As javascript runs and comes across a function, that function is added to the call stack. Javascript executes the function, adding any contained function calls further up the stack (stacking them up).
When javascript is finished with the function execution it removes the function from the call stack and continues with the rest of the code base.

Question 12

What is the “call stack” in relation to JavaScript code?

Answer 12

The call stack is how the Javascript interpreter keeps track of its location within a script that calls multiple functions, what function is currently being run, and which functions are called from within that function

Question 13

How many arguments does an event listener take?

Answer 13

An event listener only takes one argument - the event object

Question 14

What happens to an event object in an event listener?

Answer 14

The event object is passed to the listener and the return value is ignored.

Question 15

If you change a variable value within an event listener function, is that change available outside of the function?

Answer 15

No. This change is not available outside of the event listener function because by the time the event listener would execute, the scope in which it was defined would have already finished executing.

Question 16

What is currying in regards to how we write our code?

Answer 16

Currying is a paradigm of functional programming in which functions are created in such a way that they accept one argument and have no side-effects.

Question 17

How do we curry a function that would accept multiple arguments?

Answer 17

To curry a function, we nest multiple functions, each that take a single argument, and that in turn pass that argument onto further nested functions.

Question 18

Why do we curry functions in Javascript?

Answer 18

Currying allows us to create code in which it is difficult to pass less than the required arguments into a function, making the code easier to debug.

Question 19

What does curring rely on to make variables from nested functions available to each other?

Answer 19

Currying relies on the concept of lexical scope in order for variables to be available to one another.

Question 20

Where are let and const variables hosited to in JavaScript?

Answer 20

let and const are hoisted to the top of their parental scope in Javascript.

Question 21

Why is Set (as in new Set) useful in Javascript?

Answer 21

new Set will create an array of unique values from another array.

Question 22

How do we destructure an object into variables

Answer 22

we use a variable declaration (let or const) with curly braces holding the variable names that correspond to the object properties, and set that whole equal to the object name.

const {name, year, actor} = favoriteFilm

Question 23

What are the five parts of the Event Loop?

Answer 23

1. Memory Heap
2. Call Stack
3. Event Queue
4. Event Loop
5. Node / Web API’s

Question 24

What is the Heap used for in regards to JavaScript?

Answer 24

The Heap is a block of memory in which variables and objects are stored in an unordered manner.

Question 25

What is the Call Stack used for in JavaScript?

Answer 25

The Call Stack tracks what function is currently being run.

Question 26

What happens when we invoke a function in regards to the Call Stack?

Answer 26

When a function is envoked, a frame is added to the Call Stack. The frame connects that function’s arguments with local variables from the Heap. Once the function runs, and is removed from the Call Stack, the frame is cleared.

Question 27

What is the Event Queue in JavaScript?

Answer 27

The Event Queue holds a list of messages corresponding to functions that are waiting to be processed.

No code is executed in the Event Queue.

Question 28

What is the Event Loop in regards to JavaScript?

Answer 28

Messages waiting in the Event Queue are added to the Stack via the Event Loop.

Question 29

What is the Memory Life Cycle in regards to JavaScript?

Answer 29

The Memory LIfe Cycle is the processing, allocation, using, and releasing of memory in regards to program execution in JavaScript.

Question 30

What is the primary issue in regards to memory in JavaScript?

Answer 30

Memory leaks, or when things are retained in memory longer than needed.

Question 31

What are the two data structures used for memory in JavaScript?

Answer 31

The Heap

The Stack

Question 32

What type of memory objects is the Stack used for in JavaScript?

Answer 32

The Stack is used for static storage, where the size of an data structure is known when the code is compiled.

Primitive values, references to non-primitive values, and function call frames are stored in the stack

Question 33

What types of memory objects are stored in the Heap?

Answer 33

The Heap provides dynamic memory storage for data structures that don’t have a fixed size, like objects and functions.

While the object itself is stored in the Heap, the reference to the object is stored in the Stack.

Question 34

What is a memory leak in JavaScript?

Answer 34

A memory leak is when memory, that is not longer needed by the program, persists.

Question 35

What are common causes of memory leaks in Javascript code?

Answer 35

1. Messy Closures
2. Dangling Timers and Event listeners
3. Circular References
4. Declaring Variables on the global object
   5.

Question 36

How does JavaScript use the Stack to manage memory?

Answer 36

The stack is used for static stoage, where the size of an object is know when the code is compiled.

Question 37

How does JavaScript use the Heap for memory management?

Answer 37

The heap provides dynamic memory allocation at runtime. Data types that don’t have a fixed size, like objects and functions, are stored in the heap.

Question 38

How does the reference-counting algorithm help with memory management in JavaScript?

Answer 38

If a reference count for an object drops to zero, there are no more references to that object in the program and it can be discarded.

Question 39

How does Mark and Sweep help with memory management in JavaScript?

Answer 39

From the root the algorithm sweeps across the code to find anything reachable by traversing the code from all variables. Anything that is not reachable is discarded