Senior JS

Question 1

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

Answer 1

In JavaScript, blocks are denoted by curly braces {} , for example the if else, for, do while, while, try catch and so on:

Question 2

What does block scope mean?

Answer 2

variable definition is only valid within the block of code that it was declared 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 can be reassigned and const can’t

Question 5

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

Answer 5

It is adding to the array instead of reassigning the variable

Question 6

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

Answer 6

If the variable does not need to to reassigned, use const

Question 7

What is the syntax for writing a template literal?

Answer 7

Put it inside backticks, ${} for expressions and variables

Question 8

What is “string interpolation”?

Answer 8

String formatting: the ability to substitute part of the string for the values of variables or expressions

Question 9

What is destructuring, conceptually?

Answer 9

Taking out part of an object and assigning it to new variables

Question 10

What is the syntax for Object destructuring?

Answer 10

const {
      title,
      author,
      libraryID
    } = book1

Question 11

What is the syntax for Array destructuring?

Answer 11

const [book3, book4, book5] = getBooks()

Question 12

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

Answer 12

the side of the assignment operator that the object/array is on

Question 13

What is the syntax for defining an arrow function?

Answer 13

(parameters) => code block

() not needed with one parameter
() are needed with no parameter

{} needed for more than one line of code

Question 14

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

Answer 14

You don’t get a return statement

Question 15

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

Answer 15

This is defined at definition time for arrow functions.

For other functions, this is determined at call time

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.

Question 17

How do you handle the fulfillment of a Promise?

Answer 17

attach a then method handler with callback function with success message

Question 18

How do you handle the rejection of a Promise?

Answer 18

attach a catch method handler with callback function with error message

Question 19

What is Array.prototype.filter useful for?

Answer 19

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

Question 20

What is Array.prototype.map useful for?

Answer 20

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

Question 21

What is Array.prototype.reduce useful for?

Answer 21

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

Question 22

What is “syntactic sugar”?

Answer 22

making code easier to read

Question 23

What is the typeof an ES6 class?

Answer 23

function

Question 24

Describe ES6 class syntax.

Answer 24

class Stopwatch {
  constructor(elapsedSeconds) {
    this.elapsedSeconds = elapsedSeconds;
  }

tick() {
this.elapsedSeconds++;
}

  getTime() {
    const secondsPerHour = 3600;
    const secondsPerMinute = 60;
    let seconds = this.elapsedSeconds;
    let hours = Math.floor(seconds / secondsPerHour);
    seconds -= (secondsPerHour * hours);
    let minutes = Math.floor(seconds / secondsPerMinute);
    seconds -= (secondsPerMinute * minutes);
    hours = hours.toString().padStart(2, '0');
    minutes = minutes.toString().padStart(2, '0');
    seconds = seconds.toString().padStart(2, '0');
    return `${hours}:${minutes}:${seconds}`;
  }

reset() {
this.elapsedSeconds = 0;
}
}

Question 25

What is “refactoring”?

Answer 25

making code more efficient without changing its behavior

Question 26

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

Answer 26

It returns the function

Question 27

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

Answer 27

assigns the return of the value function being called twice to the const wrap variable

Not sure, look at video

Question 28

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

Answer 28

Closures