Lesson 0.6 · JavaScript in general

Conditions and logical operators

Truthy and falsy, the ternary, &&, ||, ??, ?., switch and early return.

Before we start: railway switches

A program runs like a train on rails – line after line. Conditions are the switches: depending on a question, they send the train one way or the other. "Is the user signed in?" "Is the cart empty?" "Has the data arrived from the server?"

JavaScript has two groups of tools for switches. Statements (if, switch) control which lines run. Operators (? :, &&, ||, ??, ?.) pick a value right inside an expression – and those are the ones you'll meet most in React, because you can't write an if inside JSX.

1. if, else and early return

const age = 17

if (age >= 18) {
  console.log('Adult')
} else if (age >= 13) {
  console.log('Teenager')
} else {
  console.log('Child')
}

// The one-line form – without braces only for a single statement
if (age < 0) return

Early return instead of a pyramid

When a function has to check several things, don't nest the conditions inside each other. Deal with the exceptions first and stop, and leave the main case for the end:

// ❌ A pyramid
function checkout(cart, user) {
  if (user) {
    if (cart.length > 0) {
      if (user.address) {
        return sendOrder(cart, user)
      } else {
        return 'The address is missing'
      }
    } else {
      return 'The cart is empty'
    }
  } else {
    return 'Please sign in'
  }
}

// ✅ Early return – reads top to bottom like a checklist
function checkout(cart, user) {
  if (!user) return 'Please sign in'
  if (cart.length === 0) return 'The cart is empty'
  if (!user.address) return 'The address is missing'
  return sendOrder(cart, user)
}

2. Truthy and falsy

When JavaScript needs a yes/no (in an if, a ternary, with &&) and gets something other than a boolean, it converts it. There are only six false (falsy) values – everything else is true (truthy):

Falsy – counts as NO	Truthy – counts as YES (even when it doesn't look like it)
`false`	`true`
`0`	`42`, `-1`, `0.5`
`''` (empty text)	`'0'`, `'false'`, `' '` (a space)
`null`	`[]` (an empty array)
`undefined`	`{}` (an empty object)
`NaN`	every function, every date…

if ('hello') { … }       // runs
if ('') { … }            // doesn't run
if (0) { … }             // doesn't run
if ([]) { … }            // ⚠️ runs – an empty array is truthy!

if (items.length > 0)    // ✅ the right question "does the array contain anything?"
if (items.length)        // works too (0 is falsy), but it's less clear what you're asking
if (Object.keys(obj).length > 0)   // "does the object have any properties?"

Boolean('hello')         // true – conversion to a boolean
!!'hello'                // true – the same as a shortcut (double negation)

3. Logical operators: and, or, not

const age = 25
const hasTicket = true

age >= 18 && hasTicket     // true – AND: both must hold
age < 18 || hasTicket      // true – OR: one is enough
!hasTicket                 // false – NOT: flips the value

// Combinations – parentheses for safety and readability
const canEnter = (age >= 18 && hasTicket) || isVip
const isWorkday = day !== 'saturday' && day !== 'sunday'
const isWeekend = ['saturday', 'sunday'].includes(day)   // often easier to read

4. The ternary operator

A ternary is an if/else squeezed into an expression: condition ? valueIfYes : valueIfNo. Unlike if, it has a value, so you can store it in a variable or put it anywhere in an expression.

const label = isDone ? 'Done' : 'To do'
const price = isMember ? basePrice * 0.9 : basePrice
const greeting = `Good ${hour < 12 ? 'morning' : 'afternoon'}`
const className = `btn ${isActive ? 'btn-active' : ''}`

// The same with if – longer and needs let
let label
if (isDone) {
  label = 'Done'
} else {
  label = 'To do'
}

5. && and || for picking a value

Here's the catch: in JavaScript, && and || don't return true/false. They return one of their sides:

Expression	When a is truthy	When a is falsy
`a && b`	returns b	returns a
`a \|\| b`	returns a	returns b

'hello' && 'world'   // 'world'
'' && 'world'        // ''
0 && 'world'         // 0     ← careful, the number 0!
null && 'world'      // null

'hello' || 'default' // 'hello'
'' || 'default'      // 'default'
0 || 'default'       // 'default'  ← the zero "got lost"

// Typical use
user && user.name                  // read name only if user exists (today rather user?.name)
const name = input || 'Anonymous'  // a default value for an empty input
isLoggedIn && showWelcome()        // call it only when the condition holds

6. ?? and ?.: missing values

?? – a default value only for "nothing"

a ?? b returns b only when a is null or undefined. Zero, empty text and false are valid values for it.

const settings = { volume: 0, nickname: '', notifications: false }

settings.volume || 50          // 50 ❌ the user muted it to 0 and we turned it up to 50
settings.volume ?? 50          // 0  ✅

settings.notifications || true // true ❌ the user turned them off
settings.notifications ?? true // false ✅

settings.theme ?? 'light'      // 'light' – the property is missing → the default value

// Assignment with a default value
options.retries ??= 3          // set it to 3 only if there's nothing there

Value a	a \|\| "x"	a ?? "x"
`0`	`"x"`	`0`
`''`	`"x"`	`''`
`false`	`"x"`	`false`
`null`	`"x"`	`"x"`
`undefined`	`"x"`	`"x"`
`'hello'`	`'hello'`	`'hello'`

?. – read only if there's something to read from

const user = { name: 'Anna', address: null }

user.address.city          // 💥 TypeError: Cannot read properties of null
user.address?.city         // undefined – ?. stops and returns undefined
user.contacts?.[0]         // undefined – works with square brackets too
user.getAge?.()            // undefined – and with calling a function that may not exist

// The typical pair: read safely + a default value
const city = user.address?.city ?? 'unknown'

// Data from the server that hasn't arrived yet
const firstTitle = response?.data?.items?.[0]?.title ?? 'No items'

Demo

Operator lab

What you see: A row of values for x and, below it, what converting to a boolean, a ternary, &&, || and ?? make of each one.

Try it:

Pick 0. x && 'right' returns 0 (not false!), || replaces the zero, ?? keeps it.
Pick '' and then null. For empty text || and ?? differ, for null they don't.
Pick [] and {}. Both are truthy, even though they're "empty".

The takeaway: There are only six falsy values. && and || return one of the sides, not a boolean. ?? only reacts to null and undefined.

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7. switch and a dictionary

When you compare one value with many options, switch is clearer than a chain of ifs:

function statusLabel(status) {
  switch (status) {
    case 'new':
      return 'New order'
    case 'paid':
      return 'Paid'
    case 'shipped':
    case 'delivered':        // several cases with the same result
      return 'On its way'
    default:
      return 'Unknown status'
  }
}

In functions that just turn one value into another, a dictionary (an object) is often even shorter:

const STATUS_LABELS = { new: 'New order', paid: 'Paid', shipped: 'On its way' }
const statusLabel = (status) => STATUS_LABELS[status] ?? 'Unknown status'

const ICONS = { success: '✅', error: '❌', warning: '⚠️' }
const icon = ICONS[type] ?? 'ℹ️'

You'll see switch, on the other hand, in React reducers (lesson 5), where every action does something different, not just returns a value.

Check yourself

Quiz What does 0 && 'hello' return?

false0'hello'undefined

Quiz settings.volume is 0. What is settings.volume || 50?

050undefinedfalse

Quiz Which value is truthy?

''0[]null

Quiz user is null. What does user?.name ?? 'Guest' return?

An errornullundefined'Guest'

Warm-up

Challenge

Who is who

Warm-up

One ternary and one ??.

Task (the same text is in the comment at the top of the file)

CHALLENGE: Who is who

1. ageLabel(20) should return 'adult', ageLabel(12) 'child'. The limit is 18. Use a ternary:
   condition ? 'adult' : 'child'
2. nameOrGuest('Anna') should return 'Anna', nameOrGuest(null) 'Guest'. Use ??

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I'm done

Challenge

Safe reading and decisions

Medium Bonus challenge

Six functions that make decisions and safely read data that may be missing. Watch out for the difference between || and ?? – the tests will check it.

🔒 Bonus challenge

Medium and hard challenges are a bonus for signed-in readers. Signing up is free – just an e-mail, no password.

Summary

Early return deals with the exceptions at the start of a function and saves you from if/else pyramids.
Falsy is only false, 0, '', null, undefined and NaN. An empty array and an empty object are truthy.
A ternary a ? b : c is an if/else that returns a value. Don't nest more than one.
&& and || return one of the sides, not a boolean – 0 && x is 0.
?? fills in a default value only for null/undefined, || also for 0 and ''. You almost always want ??.
?. safely reads from a value that may be missing. Typically a?.b ?? default.
switch for many cases with different logic, a dictionary for turning one value into another.

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