Truthy and Falsy Values
In JavaScript, a truthy value is a value that is considered true when encountered in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, -0, 0n, "", null, undefined, and NaN).
1. Falsy Values
There are only 8 falsy values in JavaScript. If you memorize these, you know that everything else is truthy.
false: The boolean keyword.0: The number zero.-0: The number negative zero.0n: BigInt zero.""(or'', ``): Empty string.null: The absence of any value.undefined: The primitive value.NaN: Not a Number.
if (0) {
// This code will NOT run
}2. Truthy Values
Everything that is not on the list above is truthy. This includes some values that might seem "empty" or "false" in other languages.
"0"(a string containing zero)"false"(a string containing the text "false")[](an empty array){}(an empty object)function() {}(an empty function)
if ([]) {
console.log("Empty arrays are truthy!"); // This runs
}
if ("0") {
console.log("String '0' is truthy!"); // This runs
}3. Logical Operators and Short-Circuiting
Truthy and falsy values are crucial when using logical operators || (OR) and && (AND).
OR (||)
Returns the first truthy value, or the last value if all are falsy.
const name = "";
const displayName = name || "Guest";
console.log(displayName); // "Guest" (because "" is falsy)AND (&&)
Returns the first falsy value, or the last value if all are truthy.
const isLoggedIn = true;
const user = { name: "Alice" };
// If isLoggedIn is truthy, return user.name
const userName = isLoggedIn && user.name;
console.log(userName); // "Alice"4. Double Bang (!!)
You can convert any value to its boolean equivalent using the double NOT operator (!!).
console.log(!!"hello"); // true
console.log(!!0); // false
console.log(!!{}); // trueprogramming/javascript/vanilla/javascript programming/javascript/vanilla/type-coercion