Basic Javascript

Question 1

Basic class structure

Answer 1

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
  // Getter
  get area() {
    return this.calcArea();
  }
  // Method
  calcArea() {
    return this.height * this.width;
  }
}

Question 2

are classes hoisted?

Answer 2

no

Question 3

create new instance of class

Answer 3

const square = new Rectangle(10, 10);

Question 4

create an async function (eg to fetch something)

Answer 4

async function fName() {
  const fetchedData = await fetch('url here');
}

Question 5

default export

Answer 5

export default fName() {};

Question 6

import default

Answer 6

import varName from “./fileName”;

Question 7

named export

Answer 7

export const fName = () => {};

Question 8

import named

Answer 8

import { fName } from “./fileName”

Question 9

use Promise.all

Answer 9

return Promise.all([array, promises]).then((responses) => {
      let allDataObject = {};
      allDataObject["array"] = JSON.parse(responses[0]);
      allDataObject["promises"] = JSON.parse(responses[1]);
      return allDataObject
    });

Question 10

what does .then( … … … return x;) return?

Answer 10

it returns x wrapped in a promise

Question 11

set an item in localstorage

Answer 11

window. localStorage.setItem(‘key’, ‘string value’)

note: value must be a string

Question 12

get an item from localstorage

Answer 12

let val = window.localStorage.getItem(‘key’)

Question 13

localStorage methods

Answer 13

localStorage.setItem(‘myCat’, ‘Tom’);
const cat = localStorage.getItem(‘myCat’);
localStorage.removeItem(‘myCat’);
localStorage.clear();

Question 14

convert array/object to string

Answer 14

JSON.stringify(object)

Question 15

convert from a string back to JSON object

Answer 15

JSON.parse(stringifiedObjectorArray);

Question 16

difference between for( let i in x) and for(let i of x)

Answer 16

for ..in: iterates over enumeable properties of any object. Easiest way to loop over an OBJECT’s properties. Order not guaranteed.
Works for arrays and strings but not recommended because order not guaranteed.

for ..of: for..of is a ES2015 method for iterating over “iterable collections”.
Doesn’t work for objects.
Works well with ARRAYS and STRINGS. Also good for NodeLists

Question 17

use async and await

Answer 17

async function foo() {
   await bar();
}

Question 18

async await with try / catch

Answer 18

async function f() {

  try {
    let response = await fetch('/no-user-here');
    let user = await response.json();
  } catch(err) {
    // catches errors both in fetch and response.json
    alert(err);
  }
}

f();

Question 19

Object(dot) methods

Answer 19

Object.keys = returns array of keys 
Object.values = returns array of values
Object.entries = returns array of key-value tuples(arrays)
Object.fromEntries = makes an object from an array of key-value tuples(arrays)

Question 20

useful ES6 Array methods

Answer 20

array. forEach( (val) => { do something } )
array. map( (val) => { return something }) - returns an array of returned values
array. reduce ( (acc, val) => { return modified acc }, 0) - note 0 is acc starting value

array. filter( (val) => { return true to keep } ) - returns an array of kept values
array. includes(x) - returns true or false

Question 21

Does JS copy by value or by reference?

Answer 21

Simple data types copy by value
Arrays and Objects copy by reference

NB: copy array by value:
let arrB = […arrA]