Black & White Series

Loop

JavaScript for C or Java Programmers

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while Loop

while (<condition>)
  <block>

Tests <condition> in the cycle beginning

do
  <block>
while (<condition>)

Tests <condition> in the cycle ending

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Example 1

let week = 0
const limit = Math.trunc(Math.random() * 8)
console.log('limit: ' + limit)
while (week < limit) {
  week++
  console.log('weekday: ' + week)
} 

limit: 0

limit: 2
weekday: 1
weekday: 2

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Example 2

let week = 0
const limit = Math.trunc(Math.random() * 8)
console.log('limit: ' + limit)
do {
  week++
  console.log('weekday: ' + week)
} while (week < limit)

limit: 0
weekday: 1

limit: 2
weekday: 1
weekday: 2

---

for Loop

for (<initialization>; <condition>; <increment>)
  <block>

• <initialization>
  • executed before entering
  • usually initializes a control variable
• <condition>
  • tested on entry and every complete cycle
• <increment>
  • executed every complete cycle
  • usually increments the control variable

---

Example

const limit = Math.trunc(Math.random() * 8)
console.log('limit: ' + limit)
for (let day = 1; day <= limit; day++)
  console.log('weekday: ' + day)

limit: 0

limit: 2
weekday: 1
weekday: 2

---

for...in Loop

for (<variable> in <enumerable>)
  <block>

• Iterates for all elements of an <enumerable>
  • all items of an array
  • all fields of an object
• <variable> receives each enumeration value
  • index of an array
  • field of an object

---

Example 1

const daysOfWeek = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
for (const day in daysOfWeek)
  console.log(`weekday: ${day} - ${daysOfWeek[day]}`)

weekday: 0 - Mon
weekday: 1 - Tue
weekday: 2 - Wed
weekday: 3 - Thu
weekday: 4 - Fri
weekday: 5 - Sat
weekday: 6 - Sun

---

Example 2

const author = {
  'First Name': 'Asdrubal',
  'Last Name': 'Montequio',
  'Age': 25
}
for (const a in author)
  console.log(`${a} - ${author[a]}`)

First Name - Asdrubal
Last Name - Montequio
Age - 25

---

for...of Loop

for (<variable> of <iterable>)
  <block>

• Iterates for all elements of an <iterable>
  • all items of an array
• <variable> receives each element
  • element of an array (not its index)
• Objects are not iterable

---

Example

const daysOfWeek = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
for (const dw of daysOfWeek)
  console.log(`weekday: ${dw}`)

weekday: Mon
weekday: Tue
weekday: Wed
weekday: Thu
weekday: Fri
weekday: Sat
weekday: Sun

---

Comparing

const daysOfWeek = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
for (const day in daysOfWeek)
  console.log(`weekday: ${day} - ${daysOfWeek[day]}`)

const daysOfWeek = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
for (const dw of daysOfWeek)
  console.log(`weekday: ${dw}`)

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References

• Mozilla MDN - https://developer.mozilla.org

• Eloquent JavaScript - https://eloquentjavascript.net/

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André Santanchè

www.ic.unicamp.br/~santanch/

Web2Learn

santanche.github.io/web2learn/