What are Truthy and Falsy Values in JavaScript?

if (false)     console.log("won't run");
if (0)         console.log("won't run");
if (-0)        console.log("won't run");
if (0n)        console.log("won't run");
if ("")        console.log("won't run");
if (null)      console.log("won't run");
if (undefined) console.log("won't run");
if (NaN)       console.log("won't run");
 
console.log("None of the above printed");

// All truthy - these all print
if (true)          console.log("boolean true");
if (1)             console.log("number 1");
if (-1)            console.log("negative numbers are truthy");
if (3.14)          console.log("decimals are truthy");
if ("hello")       console.log("non-empty strings");
if (" ")           console.log("space string is truthy");
if ("0")           console.log("string zero is truthy!");
if ("false")       console.log("string 'false' is truthy!");
if ([])            console.log("empty array is truthy!");
if ({})            console.log("empty object is truthy!");
if (function(){})  console.log("functions are truthy");
if (new Date())    console.log("Date objects are truthy");
if (Infinity)      console.log("Infinity is truthy");
if (-Infinity)     console.log("-Infinity is truthy");

console.log(Boolean(1));         // true
console.log(Boolean(0));         // false
console.log(Boolean("hello"));   // true
console.log(Boolean(""));        // false
console.log(Boolean(null));      // false
console.log(Boolean(undefined)); // false
console.log(Boolean([]));        // true
console.log(Boolean({}));        // true

console.log(!!1);         // true
console.log(!!0);         // false
console.log(!!"hello");   // true
console.log(!!"");        // false
console.log(!!null);      // false
console.log(!!undefined); // false
console.log(!!NaN);       // false
console.log(!![]);        // true

const value = "hello";
// Step 1: !value  => !("hello") => false
// Step 2: !!value => !(false)   => true

const username = getUserInput();
 
// Checks if username is truthy (not empty, null, or undefined)
if (username) {
  console.log(`Welcome, ${username}`);
} else {
  console.log("Please enter a username");
}

const score = 0;
const message = score ? `Score: ${score}` : "No score yet";
console.log(message); // "No score yet" (0 is falsy!)

const user = getUser();
 
// Only accesses .name if user is truthy
const name = user && user.name;
console.log(name);

const input = "";
const value = input || "default";
console.log(value); // "default" (empty string is falsy)

const items = [];
 
// Bug: this always runs because [] is truthy
if (items) {
  console.log("Has items"); // prints even when array is empty!
}
 
// Fix: check the length
if (items.length > 0) {
  console.log("Has items");
}
 
// Also valid: check length truthiness (0 is falsy)
if (items.length) {
  console.log("Has items");
}

function displayCount(count) {
  // Bug: skips the message when count is 0
  if (count) {
    console.log(`Count: ${count}`);
  } else {
    console.log("No count provided");
  }
}
 
displayCount(0); // "No count provided" - wrong!
 
// Fix: explicit null/undefined check
function displayCount(count) {
  if (count !== null && count !== undefined) {
    console.log(`Count: ${count}`);
  } else {
    console.log("No count provided");
  }
}
 
displayCount(0); // "Count: 0" - correct!

const input = "0"; // from a form field
 
// Bug: "0" is truthy, so this runs
if (input) {
  const num = Number(input);
  console.log(num); // 0
}
 
// This is actually fine here. But be careful with:
if (input === "0") {
  // explicitly handle "0"
}

function greet(name) {
  const displayName = name || "Guest";
  return `Hello, ${displayName}`;
}
 
greet("");    // "Hello, Guest" - empty string is falsy
greet(0);     // "Hello, Guest" - zero is falsy
greet(null);  // "Hello, Guest" - intended behavior
 
// Fix: use nullish coalescing (??) to replace only null/undefined
function greet(name) {
  const displayName = name ?? "Guest";
  return `Hello, ${displayName}`;
}
 
greet("");    // "Hello, "   - empty string is preserved
greet(0);     // "Hello, 0"  - zero is preserved
greet(null);  // "Hello, Guest" - null is replaced

const config = getConfig();
 
// Relies on truthy check to avoid accessing properties of null/undefined
const theme = config && config.ui && config.ui.theme;
 
// Modern alternative: optional chaining
const theme = config?.ui?.theme;

const mixed = [0, 1, "", "hello", null, undefined, false, true, NaN, 42];
 
const truthyOnly = mixed.filter(Boolean);
console.log(truthyOnly); // [1, "hello", true, 42]

const responses = [true, false, null, "yes", 0, 1, undefined, ""];
 
const truthyCount = responses.filter(Boolean).length;
console.log(truthyCount); // 3 (true, "yes", 1)

function validateForm(fields) {
  const errors = [];
 
  if (!fields.email) {
    errors.push("Email is required");
  }
 
  if (!fields.password) {
    errors.push("Password is required");
  }
 
  // Check minimum length only if password exists
  if (fields.password && fields.password.length < 8) {
    errors.push("Password must be at least 8 characters");
  }
 
  return errors;
}

const values = [false, 0, -0, 0n, "", null, undefined, NaN];

// Explicit boolean conversion
Boolean(value)  // recommended for clarity
!!value         // common shorthand
 
// Explicit checks (safer than truthy/falsy)
value === null
value === undefined
value == null            // true for both null and undefined
typeof value === "string"
Array.isArray(value)