Javascript Hard Parts

Question 1

What are 3 main parts of Javascript engine?

Answer 1

• Thread of Execution
• Memory/Variable Environment
• Call Stack

Question 2

10. What does it mean that Javascript is Lexical or Static Scoping

Answer 2

• All the data from that scope gets backpacked

* We bring the data with us from when the function was born to wherever it goes hidden in the backpack

Question 3

9. What does NEW Keyword do?

Answer 3

• Creates empty object and assign to THIS
• Has hidden property _proto _ “ dunderscore proto”
• Create a function + Object combo
• Automatically Returns the object to global memory

Question 4

8. Arrow Function

Answer 4

• THIS is the same as where it was defined
• Arrow function always get whatever the THIS Keyword context, that what it gets
• Arrow function has implied return

Question 5

7. What is This?

Answer 5

• This refers to the object to the left of the dot in that execution context. Ex: user1.increment()
• If no dot, this defaults to the window object
• Biggest Mistake is calling a function in a method and THIS is referring to Window

Question 6

6. What is Class?

Answer 6

• It is Syntactical sugar.
• There is a prototype chain that creates a link that allows the ability to look up the chain for code
• This is similar to Object Oriented Programming and the concept of inheritance where knowledge is passed down but this is just a link to look up stuff

Question 7

5. The Cycle of Code Queues

Answer 7

• Thread of Execution runs through each lines of code
• The Event Loop checks Call Stack if it has anything to run then the global code
• Then it checks the microtask queue ( Promise-deferred functions) , then Callback Queue
• Event Loop prioritizes functions in the microtask queue over the callback queue

Question 8

4. What is Promises ?

Answer 8

• Promises are invoked when there is initiated background web browser work/functionality and a place to store data in Javscript memory
• There is a placeholder object on JavaScript
• Fetch creates a promise object ( that object contains value and hidden property) and also sets up network request in web browser feature
• OnFulfillment is the hidden property of Promises ( also onRejection)
• Promise objects will automatically trigger the attached function to run (w/ the input being the returned data)
• We will call THEN method upon completion. .THEN gets a function into the unfulfilled array (Then can be considered “Run Later” )
o Any code we want to run on the returned data must also be saved on the promise object
• Promises you that it would let you know when such browser functionalities end, exits and could also provide you a way to return values

Question 9

3. What is Asynchronous Code?

Answer 9

• Asynchronous Code means doing code out of order from when you saw it
• There is web browser feature request and at the same time in JavaScript, it returns a Promise Object which acts as a placeholder object.

Question 10

2. What is Thread of Execution?

Answer 10

• Goes through the code line by line and runs/executes each line
• When it reaches a function, it does not read the code unless the function is called and continues to next line
• JavaScript is synchronous by default and is single threaded. This means that code cannot create new threads and run in parallel.

Question 11

1. What is Closure?

Answer 11

• Functions get a new memory every run/invocation
• When a function gets called, we create a new live store of data (local memory/state) for that function’s execution context
• When the function finishes executing, its local memory gets deleted except for the returned value
• Local memory gets backpacked along with function
• If our function could hold live data between executions, this would let our function have an associated cache/persistent memory
• All the relevant data from local memory execution context is stored in that function.
• Closure allows a function to have attached a permanent memory store data
• Persistent Lexical Static Scope Reference Data AKA Backpack AKA Function’s Closure