Conditional Statements in JavaScript

Conditional Statements in JavaScript

Conditional statements let your JavaScript program make decisions. Based on whether a condition is true or false, different blocks of code will run. This is the foundation of control flow — the order in which code executes.

The if Statement

const temperature = 28;

if (temperature > 30) {
  console.log('It is hot outside.');
} else if (temperature > 20) {
  console.log('It is warm outside.'); // this runs
} else {
  console.log('It is cold outside.');
}

Truthy and Falsy Values

JavaScript evaluates non-boolean values as either truthy or falsy in conditional contexts. The following six values are falsy: false, 0, '' (empty string), null, undefined, and NaN. Everything else is truthy, including empty arrays and empty objects.

if ('hello') console.log('Truthy');    // runs
if (0) console.log('Truthy');          // does not run
if ([]) console.log('Arrays are truthy'); // runs

The switch Statement

Use switch when you have multiple possible values for the same variable. Each case must end with break to prevent fall-through.

const day = 'Monday';

switch (day) {
  case 'Monday':
    console.log('Start of the work week.');
    break;
  case 'Friday':
    console.log('Last day of the work week.');
    break;
  case 'Saturday':
  case 'Sunday':
    console.log('Weekend!');
    break;
  default:
    console.log('Midweek day.');
}

Short-Circuit Evaluation

Logical operators can be used as concise conditionals. The && operator runs the right side only if the left side is truthy. The || operator runs the right side only if the left side is falsy.

const isAdmin = true;
isAdmin && console.log('Welcome, admin!'); // prints

const name = '' || 'Anonymous';
console.log(name); // 'Anonymous'