ES6 JavaScript

Question 1

What is a code block? What are some examples of a code block?

Answer 1

A block of code to be executed denoted by { curly braces }. Code blocks are found inside conditionals, loops, functions

Question 2

What does block scope mean?

Answer 2

Any variables declared inside a code block only exist within that code block.

Question 3

What is the scope of a variable declared with const or let?

Answer 3

both are block-scoped

Question 4

What is the difference between let and const?

Answer 4

Const can’t be reassigned, where let can be reassigned.

Question 5

Why is it possible to .push() a new value into a const variable that points to an Array?

Answer 5

Because we can mutate the elements of a mutable object like an array or plain object that is assigned to a const variable, and the array/object is not being reassigned.

Question 6

How should you decide on which type of declaration to use?

Answer 6

Use the const keyword when defining a variable that should not be changed - use the let keyword where you intend on changing the value. Prefer const over let.

Question 7

What is destructuring, conceptually?

Answer 7

Destructuring allows to unpack elements from an array and properties from objects and assign each to a variable.

Question 8

What is the syntax for Object destructuring?

Answer 8

const { propertykeys: variableNames} = objectName
OR
const { propertyKeys, …} = objectName (if variable name = property name)

Question 9

What is the syntax for Array destructuring?

Answer 9

const [ variableNames, … ] = arrayName

Question 10

How can you tell the difference between destructuring and creating Object/Array literals?

Answer 10

object/array literal names are on the left when creating, on the right when destructuring

Question 11

What is the syntax for writing a template literal?

Answer 11

A Template Literal uses backticks instead of single or double quotes. To insert a variable write a JavaScript Expression – ${variable}

Question 12

What is “string interpolation”?

Answer 12

The ability to substitute part of the string for the values of variables or expressions.

Question 13

What is the syntax for defining an arrow function?

Answer 13

cont variable = (param1, param2…) = > { code block}

Question 14

When an arrow function’s body is left without curly braces, what changes in its functionality?

Answer 14

If there are no curly braces there needs to be an expression following the arrow (rather than a statement) and the value of the expression is returned automatically (without return statement).

Question 15

How is the value of this determined within an arrow function?

Answer 15

An arrow function captures the This value of the enclosing context instead of creating its own this context. the This is determined at definition time (rather than at call time like regular functions)

Question 16

What are the three states a Promise can be in?

Answer 16

1. Pending: initial state, neither fulfilled nor rejected.
2. Fulfilled: meaning that the operation was completed successfully.
3. Rejected: meaning that the operation failed.

Question 17

How do you handle the fulfillment of a Promise?

Answer 17

.then ( ) method

.then (onFulfilledCallback [, onRejectedCallback] )

Question 18

How do you handle the rejection of a Promise?

Answer 18

.catch( ) method OR .then( ) method

.catch (onRejectedCallback ) OR .then(fulfilledCallback, rejectedCallback )

Question 19

What is “syntactic sugar”?

Answer 19

Syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. Does not change the behavior of the code.

Question 20

What is the typeof an ES6 class?

Answer 20

function

Question 21

Describe ES6 class syntax.

Answer 21

Use the class keyword followed by the name of the class, followed by curly braces. Inside the curly braces you can include an optional constructor function where you can add the properties of an instance and you can also add optional prototype method definitions.

Question 22

What is “refactoring”?

Answer 22

The process of restructuring existing computer code—changing the factoring—without changing its behavior.

Refactoring is intended to improve the design, structure, and/or implementation of the software, while preserving its functionality.