JS ES6

Question 1

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

Answer 1

{ }

code in curly brace

Question 2

What does block scope mean?

Answer 2

scope of variables is only within curly braces

exist within the block and only affect code within the block; can see out but cannot see in

Question 3

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

Answer 3

block scoped

Question 4

What is the difference between let and const?

Answer 4

let: mutable variable
const: immutable variable, must be initialized with value, cannot use in imperative for loop (only for…of loop)

Question 5

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

Answer 5

You can modify the values of properties, but not reassign a whole new array to a const variable (can’t change memory address of the array);

Question 6

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

Answer 6

Will the variable need to be changed within the code block?

Try to use const for everything. change to let if needed

use “const” if ARRAY, OBJECT
use “let” if LOOPS where variable is being modified

Question 7

What is the syntax for writing a template literal?

Answer 7

string ${variable}

backticks create template literals
${ } for substitutions

Question 8

What is “string interpolation”?

Answer 8

Using backticks and can substitute part of a string with values of variables or expressions

Question 9

What is destructuring, conceptually?

Answer 9

destructure individual properties of an object or elements of an array into individual variables.

Question 10

What is the syntax for Object destructuring?

Answer 10

const { property1: variable1, property2: variable2 } = object;

Question 11

What is the syntax for Array destructuring?

Answer 11

const [x, y ,…args] = arrayName

Question 12

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

Answer 12

the brackets are on the other side
creating = right side of equal sign
destructuring = left side of equal sign

Question 13

What is the syntax for defining an arrow function?

Answer 13

const add() => { }

const add = (x, y) => x + y;

OR BLOCK SYNTAX
const add = (x, y) => { return x + y; };

Question 14

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

Answer 14

without curly brace = [implicit] return, no RETURN keyword needed (anything after arrow will be evaluated and auto returned)

with curly brace = [explicit return], works like regular function code block, need RETURN keyword

Question 15

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

Answer 15

captures [this] value of enclosing context

Question 16

What are the three states a Promise can be in?

Answer 16

PENDING: initial state, neither fulfilled nor rejected.

FULFILLED: meaning that the operation was completed successfully.

REJECTED: meaning that the operation failed.

***promise always starts pending, either fulfilled/rejected (once fulfilled or rejected, cannot change)

Question 17

How do you handle the fulfillment of a Promise?

Answer 17

Promise.then(resolve (fulfillment), reject (rejection))

Question 18

How do you handle the rejection of a Promise?

Answer 18

Promise.catch

Question 19

Promise rejection - how to handle?

Answer 19

handle in .then 2nd arg

handle in .catch

Question 20

What is “syntactic sugar”?

Answer 20

syntax that is easier to read

Question 21

What is the typeof an ES6 class?

Answer 21

object

Question 22

Describe ES6 class syntax.

Answer 22

class ClassName {
   constructor(params) {
     this.params = params
    }

method( ) {
return something;
}

}

Question 23

What is refactoring?

Answer 23

Restructuring code without changing the behavior

Question 24

How are ES Modules different from CommonJS modules?

Answer 24

Common: require and module.exports

ES: import and export

Question 25

What kind of modules can Webpack support?

Answer 25

alot?

Question 26

CLOSURES

What must the return value of myFunction be if the following expression is possible?

myFunction()();

Answer 26

it needs to return a function

if you call this fxn, you are immediately calling another function and invoking the return fxn

Question 27

What does this code do?

const wrap = value => () => value;

Answer 27

wrap is a fxn that returns a fxn

wrap( ) returns the function [ ( ) => value ]

Question 28

In JavaScript, when is a function’s scope determined;

when it is called or when it is defined?

Answer 28

when it is called

Question 29

What allows JavaScript functions to “remember” values from their surroundings?

Answer 29

closures due to the lexical scope (it pulls from the scope it was defined in)