ES6

Question 1

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

Answer 1

Piece of code that is to be ran.

Function code block
Conditional code block
Loop code block

Question 2

What does block scope mean?

Answer 2

The area inside a code block.

Question 3

What is a block scoped variable?

Answer 3

Exists inside code block only

Question 4

What is the difference between let and const?

Answer 4

let is:
mutable, not attached to the global object
Can be reassigned

const is:
immutable, read-only (cannot be changed but properties can be altered).
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

Push works because you’re not reassigning the data type of the array is but changing contents of the array.

Question 6

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

Answer 6

Use const unless you cant.

If variable will get reassigned use let.

Question 7

What is the syntax for writing a template literal?

Answer 7

Backticks ``
Strings
Expressions (using ${JavaScript expression}
- Any expression that has a value could be used

Question 8

What is ‘string interpolation’?

Answer 8

The substitution of a string using the values of expressions and variables

${expressionName}

Question 9

What is destructuring, conceptually?

Answer 9

A way to extract values from declared arrays and objects

Question 10

What is the syntax for Object destructuring?

Answer 10

variable keyword
curly braces
property names
variable names (if not using property names)
equal sign
object name

Question 11

What is the syntax for Array destructuring?

Answer 11

const [x, y, z] = arrayName

variable keyword
brackets
element names in array
variable name (if not using element name)
equal sign
array name

Question 12

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

Answer 12

Destructuring curly braces/brackets on the left side and object/array name on right side

Creating curly braces/brackets on the right side and object/array name on left side

Question 13

What is the syntax for defining an arrow function?

Answer 13

(p1, p2, …, pn) => expression;

Variable (optional dependent on use case)
Parameters in parentheses (optional)
Arrow function
Code block or expression

Question 14

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

Answer 14

Without curly brace return is implied

With curly brace return statement is needed

Question 15

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

Answer 15

With whatever this was assigned to

Question 16

What is a Promise?

Answer 16

A proxy value that isn’t known when the promise is made

Question 17

What are the three states a Promise can be in?

Answer 17

Pending: the initial state, neither fulfilled or rejected
Fulfilled: a successful operation
Rejected: a failed operation

Question 18

How do you write a then method?

Answer 18

const promiseName = return of promise function

promiseName.then(parameterName => {
console.log(parameterName);
});

parameterName is defined as the fulfillment of the promise

Question 19

How do you write a catch method?

Answer 19

const promiseName = return of promise function

promiseName.catch(error => {
console.error(error.message);
});

error is defined as what happens when the promise is rejected

Question 20

How do you handle a fulfilled promise?

Answer 20

Using a then method and passing a callback

which contains the fulfilled argument

Question 21

How do you handle a rejected promise?

Answer 21

Using the catch method and using a callback function with the rejected argument

Question 22

What is Array.prototype.filter useful for?

Answer 22

Creating a new array that has values that pass the test implemented by the function.

Question 23

How to write an Array.prototype.filter()?

Answer 23

const filteredArray = array.filter(predicate => {
code block
});

Question 24

What is Array.prototype.map useful for?

Answer 24

A new array with each element from the array being the result of the function being used.

Question 25

How to write Array.prototype.map?

Answer 25

const mappedArrayName = array.map(arrayValue => {
code block
});

Question 26

If you didn’t have the map method how would you write it?

Answer 26

function map(array, transform) {
   const mapped = [ ];
   for (let i = 0; i < array.length; i++) {
      mapped.push(transform(array[i]));
   }
   return mapped;
}

Question 27

Why are for loops faster than methods?

Answer 27

Methods have to go through stack of an event loop

Question 28

If you didn’t have filter method how would you have to write it?

Answer 28

function filter(array, predicate) {
   const filtered = [ ];   
   for (let i = 0; i < array.length; i++) {
      if (predicate(array[i])) {
         filtered.push(array[i]);
      }
   }
   return filtered;
}

Question 29

What is Array.prototype.reduce useful for?

Answer 29

To combine elements in an array into a single value

Question 30

How to write an Array.prototype.reduce?

Answer 30

const reducedArray = array.reduce([previousValue, currentValue  => {
codeblock
});

Name previousValue the return value of what your variable will be named
Name currentValue an element from your array

Question 31

If reduce method did not exist how would you write a raw function of it?

Answer 31

function reduce(array, combine, initialValue) {
   var i = 0;
   if (arguments.length === 2) {

}
}

Question 32

What is “syntactic sugar”?

Answer 32

Syntax that makes the programming language easier to read/express.

Question 33

What is typeof an ES6 class?

Answer 33

Function

Question 34

Describe ES6 class syntax.

Answer 34

class className {
   constructor(constructorName) {
      this.constructorName = constructorName;
   }
   getConstructorName( ) {
      return this.constructorName;
   }
}

• Constructor part isn’t necessary

Question 35

What is “refactoring”?

Answer 35

Process of restructuring existing code without changing the original functionality. This in turn improves:

• Readability
• Reduces complexity
• Faster performance
• Less memory usage

Question 36

How are ES modules different from CommonJS modules?

Answer 36

ES modules are different because they use export with the defined javascript expression name, export default, or export with export names in curly brace.

Does not support require for imports or module.exports

ES is officially a part of JavaScript and CommonJS isn’t. However webpacks supports both

Question 37

What kind of modules can Webpack support?

Answer 37

ECMAScript
CommonJS
AMD
Assets
WebAssembly