JavaScript

Question 1

When we run our code, what is created?

Answer 1

The Global Execution Context

Question 2

The Global Execution Context consists of which two things?

Answer 2

1) Thread of Execution

2) Global Variable Environment or Global Memory

Question 3

When we execute a function, what is created?

Answer 3

The Local Execution Context or the Function Execution Context

Question 4

What is the Thread of Execution?

Answer 4

It parses and executes the code line by line

Question 5

The Local Execution Context consists of which two things?

Answer 5

1) Thread of Execution

2) Variable Environment or Local Memory

Question 6

What is the Variable Environment?

Answer 6

It is live memory of variables with data in the function

Question 7

What is the Global Variable Environment?

Answer 7

It is live memory of variables with data

Question 8

What is the first item on the Call Stack?

Answer 8

The Global Execution Context

Question 9

When is an item pushed onto the Call Stack?

Answer 9

When a function is called, it is ‘pushed’ on the stack. When it is finished then it is ‘popped’ off the stack.

Question 10

What is a function parameter?

Answer 10

The name listed in the function’s definition

Question 11

What is a function argument?

Answer 11

The real value passed into the function

Question 12

What are the primitive types in JavaScript?

Answer 12

strings, numbers, booleans, undefined, and null

Question 13

What is stored in the Stack?

Answer 13

Primitives and References because they are fixed size. The size is known at compile time. This is static memory allocation.

Question 14

What is stored in the Heap?

Answer 14

Objects and Functions. The size is known at runtime. This is dynamic memory allocation.

Question 15

Do variables point at the Stack or the Heap?

Answer 15

All variables point at the Stack. For objects and functions, the Stack has a reference address to find them in the Heap.

Question 16

What JavaScript engine does Chrome use?

Answer 16

V8

Question 17

What JavaScript engine does Node.js use?

Answer 17

V8

Question 18

What JavaScript engine does Firefox use?

Answer 18

SpiderMonkey

Question 19

What JavaScript engine does Safari use?

Answer 19

JavaScriptCore

Question 20

What JavaScript engine does Edge use?

Answer 20

V8, but used to be Chakra

Question 21

What is an IIFE?

Answer 21

Immediately Invoked Function Expression. It is a self-executing anonymous function.

Question 22

What is a variable?

Answer 22

It is a named container for storing values

Question 23

What are variable names called in JS?

Answer 23

Identifiers

Question 24

What is scope?

Answer 24

Scope defines the lifetime and visibility of a variable.

Scope === Variable Access

Question 25

What is the global object?

Answer 25

The global object is an object that always exists in the global scope.

1) Web Browsers have the ‘window’ object
2) Node has the ‘global’ object

Question 26

What are the different kinds of scope?

Answer 26

1) Global scope
2) Function scope
3) Block scope
4) Module scope

Question 27

What is Global scope?

Answer 27

Variables defined outside of any function, block, or module have Global scope. Variables in Global scope can be accessed anywhere in the application. Global variables are available for the lifetime of the application.

Question 28

What is Function scope?

Answer 28

Variables defined in a function are visible everywhere within a function, but not visible outside the function. Function scope is only applicable to variables declared with the ‘var’ keyword.

Question 29

What is Block scope?

Answer 29

It is defined with curly braces. It could be functions, loops, if statements, etc. Block scope is only applicable to variables declared with the keywords ‘let’ or ‘const’.

Question 30

What is Module scope?

Answer 30

Variables defined inside a module are hidden and not available to any other module unless it is explicitly exported.

Question 31

What is Lexical scope?

Answer 31

It is when a child scope has access to the parent scope.

Question 32

What is scope chain?

Answer 32

Every scope has a link to its parent scope. When a variable is used, it looks down the scope chain until it finds the requested variable or it reaches the Global scope.

Question 33

What is a closure?

Answer 33

A closure is an object that combines two things: a function, and the environment in which the function was created. The environment contains any local variables that were in-scope at the time the closure was created.

Question 34

What is Hoisting?

Answer 34

Hoisting is a mechanism where variable and function declarations are moved to the top of their scope before code execution.

Question 35

What happens when Hoisting a variable?

Answer 35

When ‘var’ is hoisted to the top of its scope, it is declared and initialized to undefined.
When ‘let’ and ‘const’ are hoisted to the top of their scope, they are declared but inaccessible until their formal declaration.

Question 36

What is the Temporal Dead Zone?

Answer 36

This is in reference to hoisting with ‘let’ and const’. Between their declaration are the top of the scope and their formal declaration in code, they are inaccessible, which is the Temporal Dead Zone.

Question 37

What is a function declaration?

Answer 37

A function declaration is the creation of a function starting with the function keyword. Eg. 
function greeting() {
  // Some Code
}

Question 38

What is a function expression?

Answer 38

A function expression is a function assigned to a variable. Eg.
const greeting = function() {
  // Some Code
};

Question 39

What happens when hoisting a function?

Answer 39

A function declaration is hoisted to the top of its scope and initialized with it’s function value.
A function expression’s variable is hoisted to the top of its scope, but initialized to undefined, unless it is ‘let’ or ‘const’ then it is inaccessible due to the TDZ

Question 40

What is a pure function?

Answer 40

1) A function is pure when given the same input, it will always return the same output.
2) It also produces no side effects.

Question 41

What is a Higher Order Function?

Answer 41

It is a function that takes a function as a parameter and/or returns a function.

Question 42

What are the two phases of the Global Execution Context?

Answer 42

1) Creation Phase

2) Execution Phase

Question 43

What happens in the Creation Phase for the Global Execution Context?

Answer 43

1) Create the global object (eg. window in browser)
2) Initializes the value of ‘this’ (which is equal to global object)
3) Set aside memory space for variables (Set undefined)
4) Store function declarations in memory

Question 44

What happens in the Execution Phase for the Global Execution Context/Function Execution Context?

Answer 44

It executes the code line by line

Question 45

What happens in the Creation Phase for the Function Execution Context?

Answer 45

1) Create the arguments object
2) this keyword
3) Set aside memory space for variables
4) Set parameter(s) to argument value(s)

Question 46

What is the Event Loop?

Answer 46

The event loop handles asynchronous tasks. It is implemented by the browser or by nodejs, and not by the JavaScript engine. It picks up tasks from the message queue or job queue.

Question 47

What is the message queue?

Answer 47

When asynchronous tasks, called by the browser’s API, are completed, they are placed in the message queue waiting to be picked up by the event loop. They will be placed on the call stack as soon as it is empty.

Question 48

What is the job queue?

Answer 48

The job queue is for micro tasks. Promises and Async/Await get to skip the message queue and will be executed first.

Question 49

What is the ‘this’ keyword?

Answer 49

The ‘this’ keyword represents the object that is executing the current function.

1) method -> ‘this’ represents the object the method is being called on.
2) function declaration -> ‘this’ represents the global object
3) arrow function -> ‘this’ represents the object the function is called on

Question 50

What is array destructuring?

Answer 50

let [one, two] = [true, false];

Brackets for both, and order indicates assignment

Question 51

What is object destructuring?

Answer 51

let { first, second } = { first: 1, second: 2 };

Braces for both, and name/label indicates assignment

Question 52

How to swap variables without using a temp?

Answer 52

let a = 1, b = 2; 
[b, a] = [a, b];

Question 53

What is the spread operator?

Answer 53

let a = [1, 2, 3];
let b = [4, 5, 6];
let c = […a, …b];

It takes an array of values of spreads them across arguments or elements.

Question 54

What is the rest operator?

Answer 54

The rest parameter allows us to pass an indefinite number of parameters to a function and access them in an array.

Question 55

When do you use let?

Answer 55

Same as var but has scope implications. eg. Block scope vs function scope (var)

Question 56

When do you use const?

Answer 56

When you don’t want a primitive to change value, or you don’t want an object to change type.

Question 57

What is the void operator?

Answer 57

It evaluates the given expression and then returns undefined. eg. void 0 returns undefined.

Question 58

What is the delete operator?

Answer 58

It removes a property from an object.

It removes an element from an array, but the length remains the same.

Question 59

How is a generator defined?

Answer 59

It is defined with an asterisk. Calling a generator returns a Generator object. 
eg. function* generator(i) {
  yield i;
  yield i + 10;
}

Question 60

What is the yield keyword?

Answer 60

It is used to pause and resume a generator function.

Question 61

What is the get keyword?

Answer 61

It can be used on an object to create a getter. This is a function to create a computed property. It is a function but accessed like a property.