djD-REK

// With no type checks, the code seems to work fine.
const user1 = { id: 1, name: "Erowyn" }
const user2 = { id: 2, name: "Lavendula" }
const user3 = { id: 3, name: "Fralia" }
const userArray = [user1, user2, user3]
for (const user of userArray) {
  console.log(`${user.id}: ${user.name}`)
}
// Output:
// 1: Erowyn

djD-REK

const object = { "🔑key🔑": "💸value💸" }
console.log(typeof object) // "object"
console.log(object !== null) // true
console.log(object != null) // true

console.log(typeof null) // "object"
console.log(null !== null) // false
console.log(null != null) // false

console.log(typeof undefined) // "undefined"

djD-REK

// && can be used to prevent errors, because the error won't be thrown.
const nullBanana = null
try {
  console.log(nullBanana.length)
} catch (e) {
  console.log(e)
}
// Output: "TypeError: null has no properties."

// Using && short-circuits the code, so the TypeError doesn't occur.

djD-REK

// Using && will prevent nonsense code from being executed.
false && console.log(NoNsEnSe_CoDE) // nothing happens

// You'll frequently see && used for type-checking.
const bananas = "🍌🍌🍌🍌🍌"
typeof bananas === "string" && console.log(bananas) // 🍌🍌🍌🍌🍌

// For example, you might be expecting an array, not a string.
Array.isArray(bananas) && bananas.push("🍌") // nothing happens

djD-REK

// The typeof null is "object" in JavaScript.
console.log(typeof null === "object") // true

// Let's look at type checking a Date object.
const date = new Date()
console.log(typeof date) // "object"
console.log(Object.prototype.toString.call(date)) // [object Date]
console.log(date.constructor) // function Date()
console.log(date.constructor.name) // "Date"
console.log(date instanceof Date) // true

djD-REK

const addWeirdStuffMap = (arrayOne, arrayTwo) => {
  const [sumOfEvensInArrayTwo, sumOfOddsInArrayTwo] = arrayTwo.reduce(
    ([sumOfEvens, sumOfOdds], itemTwo) => {
      return itemTwo % 2 === 0
        ? [sumOfEvens + itemTwo, sumOfOdds]
        : [sumOfEvens, sumOfOdds + itemTwo]
    },
    [0, 0] // Start with an initial value of 0 for both sums
  )
  // Add one to each itemOne if arrayTwo contains an element greater than 20

djD-REK

const addWeirdStuffRefactor = (arrayOne, arrayTwo) => {
  const sumOfEvensInArrayTwo = arrayTwo.reduce(
    (sumOfEvens, itemTwo) =>
      itemTwo % 2 === 0 ? sumOfEvens + itemTwo : sumOfEvens,
    0 // Start with an initial value of 0
  )
  const sumOfOddsInArrayTwo = arrayTwo.reduce(
    (sumOfOdds, itemTwo) =>
      itemTwo % 2 === 1 ? sumOfOdds + itemTwo : sumOfOdds,
    0 // Start with an initial value of 0

djD-REK

const addWeirdStuff = (arrayOne, arrayTwo) => {
  let sumOfEvensInArrayTwo = 0
  let sumOfOddsInArrayTwo = 0
  let addOneToArrayOne = 0 // if an element of arrayTwo is over 20
  for (const itemTwo of arrayTwo) {
    if (itemTwo % 2 === 0) {
      sumOfEvensInArrayTwo += itemTwo
    }
    if (itemTwo % 2 === 1) {
      sumOfOddsInArrayTwo += itemTwo

djD-REK

// Enter these commands into the console to load jQuery first.
// <script src="https://code.jquery.com/jquery-3.5.0.js"></script>
const scriptSource = "https://code.jquery.com/jquery-3.5.0.js"
document.write(`<script src="${scriptSource}" type="text/javascript"></script>`)

// Then, enter the following commands separately:
console.log($) // function jQuery(selector, context)
console.log(typeof $) // "function"

console.log($()) // Object {  }

djD-REK

console.log($) // function()
console.log(typeof $) // "function"

console.log($()) // null
console.log(typeof $()) // "object"

try {
  console.log($().jquery)
  console.log(typeof $().jquery)
} catch (e) {