JavaScript ES6

Question 1

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

Answer 1

code block is a group of zero or more statements to be executed once it meets a condition. Examples: if else, for, do while, while, try catch.

Question 2

What does block scope mean?

Answer 2

The scope created with a pair of curly braces

Question 3

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

Answer 3

blocked-scope

Question 4

What is the difference between let and const?

Answer 4

let can be reassigned. const cannot be reassigned.

Question 5

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

Answer 5

you’re not changing a property(key) value pair, you’re adding a new one onto it.

Question 6

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

Answer 6

if you know you need to change variable later, use let, otherwise use const.

Question 7

What is the syntax for writing a template literal?

Answer 7

let simple = this is a template literal; (text wrapped in backticks)

Question 8

What is “string interpolation”?

Answer 8

the ability to substitute part of the string for the values of variables or expressions with special block ${variable_name}

Question 9

What is destructuring, conceptually?

Answer 9

It assigns properties of an object to individual variables.

Question 10

What is the syntax for Object destructuring?

Answer 10

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

if same name for property and variable, more concise code

let { firstName, lastName } = person;

Question 11

What is the syntax for Array destructuring?

Answer 11

let [x, y, z] = array;

Question 12

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

Answer 12

variable of the source object/array is to the right of the assignment operator for destructuring and to the left is the values to unpack from the sourced variable.

for creating a literal, const/let is to the left of the array/object variable name.

Question 13

What is the syntax for defining an arrow function?

Answer 13

param => {
statements
}

(param1, paramN) => {
statements
}

Question 14

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

Answer 14

With this syntax, the arrow function has an implicit return. No return statement, has to be an expression.

without the curly braces, you can only have one expression

Question 15

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

Answer 15

the value of this is determined by the value of “this” in its containing code block

The arrow function retains the value of the this
In normal functions this is assigned at the value of call time
In arrow function this is being assigned at the time of definition

Question 16

What is the JavaScript Event Loop?

Answer 16

The Event Loop is one of the four major concepts that sets JavaScript apart from many other languages

Question 17

What is different between “blocking” and “non-blocking” with respect to how code is executed?

Answer 17

blocking is code that prevents further code from running until the current task is completed. it is on the stack.

non-blocking is asynchronous. it is off the stack.

Question 18

What are the three states a Promise can be in?

Answer 18

pending, fulfilled, rejected

Question 19

How do you handle the fulfillment of a Promise?

Answer 19

call the promise’s “then” method

Question 20

How do you handle the rejection of a Promise?

Answer 20

call the promise’s “catch” method

Question 21

What is Array.prototype.filter useful for?

Answer 21

The filter() method creates a new array with all elements that pass the test implemented by the provided function

Question 22

What is Array.prototype.map useful for?

Answer 22

The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.

Question 23

What is Array.prototype.reduce useful for?

Answer 23

To execute a reducer function (that you provide) on each element of the array, resulting in single output

Question 24

What is “syntactic sugar”?

Answer 24

Syntax designed to make things easier to read or to express.

Question 25

What is the typeof an ES6 class?

Answer 25

function

Question 26

Describe ES6 class syntax.

Answer 26

the keyword ‘class’ followed by the className, a constructor, and methods.

Question 27

What is “refactoring”?

Answer 27

Process of restructuring computer code without changing its external behavior. Intended to improve design/structure/implementation.

Question 28

How are ES Modules different from CommonJS modules?

Answer 28

A ES Modules syntax is more compact than CommonJS, structure can be statically analyzed, the support for cyclic dependencies is better than CommonJS

Question 29

What kind of modules can Webpack support?

Answer 29

A ECMAScript modules, AMD Modules, CommonJS

Question 30

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

Answer 30

return function

Question 31

What does this code do?
const wrap = value => () => value;

Question 32

In JavaScript, when is a function’s scope determined; when it is called or when it is defined?

Answer 32

when it is defined

Question 33

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