Conditions: if, else and switch

You make dozens of decisions every day: if it's raining, you take an umbrella; if it's late, you call a taxi; if your balance is low, you put off a purchase. Programs make decisions like these too — with the help of conditional statements.

if: when a condition is true

After the keyword if you write a condition in parentheses, and inside the curly braces { } goes the code to run when the condition is true — a block. If the condition is false, the block is skipped and the program carries on.

const temperature = 28;

if (temperature > 25) {
  console.log('It is hot. Drink some water!');
}

console.log('Have a nice day');

It is hot. Drink some water!
Have a nice day

else and else if

The else block runs when the condition is false. If there are more than two options, you add new conditions with else if. JavaScript checks the conditions from top to bottom and runs the block of the first true one — it ignores the rest.

const score = 78;

if (score >= 90) {
  console.log('Excellent');
} else if (score >= 70) {
  console.log('Good');
} else if (score >= 50) {
  console.log('Satisfactory');
} else {
  console.log('Try again');
}

Good

Combining conditions

To combine several conditions we use logical operators: && (AND) requires both conditions to be true, and || (OR) requires at least one. Suppose a cinema gives a discount to people under 18 at weekends:

const age = 15;
const isWeekend = true;

if (age < 18 && isWeekend) {
  console.log('Discount ticket: 3 AZN');
} else {
  console.log('Regular ticket: 5 AZN');
}

const day = 'Sunday';
if (day === 'Saturday' || day === 'Sunday') {
  console.log('No school today');
}

Discount ticket: 3 AZN
No school today

The ternary operator

When you just need to pick one of two values and store it, there is a short form: condition ? value1 : value2. If the condition is true, the result is value1, otherwise value2. It is called the ternary operator because it has three parts.

const hour = 20;
const greeting = hour < 12 ? 'Good morning' : 'Good evening';
console.log(greeting);

const items = 1;
console.log(`You have ${items} ${items === 1 ? 'item' : 'items'} in the basket`);

Good evening
You have 1 item in the basket

switch: many options

When you need to compare one variable with many exact values, switch is easier to read than a long else if chain. Each case checks one value (with strict comparison ===), and break exits the switch. If no case matches, the default block runs.

const day = 'Saturday';

switch (day) {
  case 'Saturday':
  case 'Sunday':
    console.log('Weekend!');
    break;
  case 'Friday':
    console.log('Almost weekend');
    break;
  default:
    console.log('Work day');
}

Weekend!

const light = 'yellow';

switch (light) {
  case 'green':
    console.log('Go');
  case 'yellow':
    console.log('Get ready');
  case 'red':
    console.log('Stop');
}

Get ready
Stop

const score = 83;
// 1) print the grade: A, B, C or F
// 2) print passed or failed with the ternary operator

B
passed