Java Script - The Advanced Concepts

Question 1

What does it mean that JavaScript is a non-blocking language?

Answer 1

It means that while it’s a single-threaded language, it can have asynchronous behavior (so that a asynchronous line doesn’t block the rest of the code).

Question 2

Is JavaScript compiled or interpreted?

Answer 2

It’s compiled through JIT compilers that try to combine aspects from compiled and interpreted languages.

Question 3

How does the JIT compiler works?

Answer 3

It checks for possible code optimizations and compile them on the fly

Question 4

Which expressions are hard to optimize ?

Answer 4

eval, arguments, for…in, with and delete

Question 5

What should you do with objects to optimize hidden classes ?

Answer 5

Add all properties in the same order for similar objects

Question 6

How does hidden classes work?

Answer 6

Every object has a hidden class. Once a new property is added, a new hidden class is created and a transition between them is stablished.

Question 7

How is the garbage collection done in JS ?

Answer 7

Mark and Sweep (mark all objects that are referenced in memory, sweep the ones that weren’t marked before).

Question 8

Which are some of the most common causes of memory leaks in JS?

Answer 8

Use of global variables, not removing obsolete event listeners.

Question 9

How can you access the methods from the web API?

Answer 9

With the “window” command.

Question 10

When is the callback queue executed ?

Answer 10

When the callstack is empty

Question 11

What is the predominant scope used in JS?

Answer 11

Static (lexical) scope

Question 12

When does hoisting happen ?

Answer 12

Every time a new execution context is created

Question 13

Which keywords are hoisted?

Answer 13

Functions are fully hoisted; vars are partially hoisted.

Question 14

What’s the most efficient way to iterate through objects?

Answer 14

Use Object.keys (obj) to get the keys in a separate array and iterating through it.

Question 15

Why are the “eval” and “with” keywords hard to optimize?

Answer 15

Because they distort how the variable scope works.

Question 16

How can you check the scope of a function ?

Answer 16

Through the [[scope]] block.

Question 17

What happens when you create a variable without a “var”, “let” or “const” identifier?

Answer 17

It is created on the global scope.

Question 18

Is the “var” keyword local, function or global scoped ?

Answer 18

function

Question 19

Is the “let” keyword local, function or global scoped ?

Answer 19

local

Question 20

What’s the syntax of IIFEs?

Answer 20

(function(…) {…} ) () ;

Question 21

Are IIFEs hoisted? Why?

Answer 21

No, because they don’t start with the “function” keyword.

Question 22

What’s the biggest advatage of IIFEs?

Answer 22

It helps keeping the global namespace clean by encapsulating all variables and functions inside of a single globally scoped variable.

Question 23

Is the “this” keyword lexically or dynamically scoped?

Answer 23

Dynamically

Question 24

Is it possible to change the scope of the “this” keyword ? How?

Answer 24

You change the scope of the “this” keyword to lexical scope when you use it inside of an arrow function.

Question 25

” ‘function (x) {…}’ is equivalent to ‘(x) => {…}’ “. True or false statement?

Answer 25

False. On the first one, “this” will have dynamic scope. In the second, it has lexical scope. The second one also doesn’t have access to the “arguments” keyword.

Question 26

What method is called when you run a function ?

Answer 26

The call ( ) method from the same function you are trying to run.

Question 27

What are the two most commonly used ways of sharing the same method between different objects ?

Answer 27

Using the “this” keyword inside the method or through method.apply( ).

Question 28

What’s the meaning of the first parameter of the call () method?

Answer 28

The object at which the function should be called upon. If null is passed, it will be called upon the window object.

Question 29

What does “function.bind()” does ?

Answer 29

It binds the “this” keyword inside the function to the object passed as a parameter.

Question 30

“Functions in JS are just objects”. True or False ?

Answer 30

True

Question 31

Is JS strongly or weakly typed ? What does it mean?

Answer 31

Weakly. It means that the language is more lenient when dealing with operations with different types

Question 32

Is JS statically or dynamically typed? What does it mean?

Answer 32

Dynamically. It means the type of the variables are defined on execution instead of compilation.

Question 33

Functions are which kind of object?

Answer 33

Callable objects

Question 34

Why are closures needed in JS?

Answer 34

They are needed to guarantee that any functions declared inside another function can have access to their variables even if the parent function was already executed and is not on memory anymore.

Question 35

What are the biggest advantages of using closures ?

Answer 35

Time and memory efficiency. They also permit the creation of private variables in a OOP context.

Question 36

What kind of inheritance is implemented in JS?

Answer 36

Prototypal Inheritance

Question 37

Which type in JS has the “prototype” property?

Answer 37

Functions

Question 38

“The reccomended way of inheriting a prototype is by changing the __proto__ property to the desired parent object”. True or False?

Answer 38

False. The preferred method is through “Object.create(parent)”

Question 39

“When checking for a property in an object, JS will start with the top of the prototype chain and follow down until the object”. True or False?

Answer 39

False. It’s the opposite direction.

Question 40

What’s the biggest advantage of prototypal inheritance over classic inheritance?

Answer 40

On classical inheritance, only classes can be inherited from classes. Prototypes can be inherited by other prototypes and objects.

Question 41

What’s the main use of prototypes in JS?

Answer 41

Simulating OOP.

Question 42

How can you create objects (in OOP sense) in JS?

Answer 42

Through “Object.create(constructor)”

Question 43

How can you add properties to constructors in JS?

Answer 43

By assigning them to the function prototype

Question 44

Which operator is used to create OOP objects in JS?

Answer 44

The “new” operator.

Question 45

“When you create two different objects from the same prototype, their functions will be pointing to the same memory address”. True or False?

Answer 45

True if the function was added to the prototype, false if it was created inside the constructor itself.

Question 46

“The only way to add variables to constructors is through the ‘this’ keyword”. True or False?

Answer 46

True

Question 47

What’s the difference between imperative and declarative programming?

Answer 47

Imperative is defining what to do and how to do. Declarative is about telling what to do and what’s the expected result.

Question 48

Which Functional Programming principle does memoization use?

Answer 48

Memoization works through caching the result of previous calls to make future calls faster. This requires the idempotence principle to work (the idea that a function should return the same output for the same input).

Question 49

How can you create private variables in JavaScript?

Answer 49

By using closures to encapsulate them.

Question 50

What does currying means?

Answer 50

It means taking a function that receives n arguments and turning it into n functions that receive one argument and return a function for the next.

Question 51

What’s the difference between currying and partial application?

Answer 51

Currying is a partial application one argument at a time. Partial application can also be used to create a function that receives only the first argument and return a function that receives all the others simultaneously.

Question 52

What’s the meaning of arity?

Answer 52

The amount of parameters a function receives

Question 53

How can you improve the arity of a function?

Answer 53

By converting it into a function that applies the argument only partially.

Question 54

When should Functional Programming be preferred to Object Oriented Programming?

Answer 54

When you are dealing with massive amounts of data.

Question 55

When should Object Oriented Programming be preferred to Functional Programming?

Answer 55

When there are many objects that could share the same methods

Question 56

What does it mean to compose functions?

Answer 56

It means to apply a function on the result of a function.

Question 57

What’s the difference between compose and pipe?

Answer 57

Composing two functions f and g means executing g and applying f to the result. Piping them means executing f and applying g to the result.

Question 58

“JavaScript is a synchronous, single-threaded, non-blocking language”. True or False?

Answer 58

True

Question 59

Which are the possible states for promises?

Answer 59

Fulfilled, Rejected or Pending

Question 60

What does it mean that a promise was settled?

Answer 60

It means that it was completed, either fulfilled or rejected.

Question 61

How is the standard way of creating promises?

Answer 61

const p = new Promise ( (resolve,reject) => {if (…) resolve() else reject()} );

Question 62

When does the execution of a promise begin?

Answer 62

Right after it’s instantiation

Question 63

What happens when you use “promise.then()”?

Answer 63

It will execute the function inside the “then()” part after it’s finished executing; the parameters passed to the function will be the return of the promise.

Question 64

What’s the type of the return of the resolve() and reject() functions?

Answer 64

Promises

Question 65

What happens when a promise gets rejected inside a chain?

Answer 65

The following “then()” promises won’t execute and the code will look if there’s any “.catch()” to handle the reject scenario

Question 66

What’s the difference between “Promise.all()” and “Promise.allSettled()”?

Answer 66

Promise.all() will only execute the following “then()” functions if all the promises inside are fulfilled. Promise.allSettled() will execute after all the promises finish execution (doesn’t matter how many, if any, gets fulfilled).

Question 67

“Async / Await is just syntatic sugar for promises”. True or False?

Answer 67

True

Question 68

“The finally block is also available with promises to clear up things whether they get resolved or rejected”. True or False?

Answer 68

True

Question 69

How can you iterate through a promise array?

Answer 69

With “for await (… of …)”.

Question 70

When are promises executed (in relation to the synchronous JS code)?

Answer 70

After the code has finished executing (but before it has returned) and the stack is empty.

Question 71

Do promises use the job queue or the callback queue?

Answer 71

Job Queue

Question 72

What’s the difference between the “job queue” and the “callback queue”? Which (if any) has priority?

Answer 72

The “job queue” (also known as micro-task queue) is for more relevant (and usually faster) operations wrapped around promises. It has priority over the callback queue (that’s used with the web API).

Question 73

How are the three ways you can execute a different group of promises?

Answer 73

Parallel (all of them running at the same time, the code returns when all of them finish executing), sequential (one after the other, code returns when the last one finishes) and race (all at the same time, code returns the result of the first one to finish).

Question 74

How are the asynchronous functions handled underneath the hood?

Answer 74

With service workers that work on different threads.