JavaScript Booleans

A boolean is the simplest data type there is: it has exactly two possible values, true and false. Booleans are the language of decisions. Every if, every loop, and every comparison comes down to one of these two values.

Programs constantly need to represent one of two states:

• yes or no (isMember)
• on or off (isMuted)
• true or false (hasPassed)

// Boolean Values

let isOnline   = true;
let isLoggedIn = false;
console.log(typeof isOnline);

Two rules for writing them:

• true and false must be lowercase. True or FALSE is not a boolean; JavaScript reads it as a variable name, and you get a ReferenceError
• true and false are written without quotes. "true" is a four-letter string, not a boolean

// Booleans from Comparisons

You rarely type true or false directly. Most booleans come from comparisons, and every comparison operator gives one back. Given let x = 5;:

You can store the result of a comparison in a variable, and use it later:

const score     = 85;
const passed    = score >= 60;
const excellent = score >= 90;

if (passed) {
  console.log("You passed! Excellent: " + excellent);
}

// Where Booleans Are Used

Booleans drive every kind of decision in JavaScript. You have seen conditions; loops (coming up in the next section) use them too, to decide whether to keep repeating:

// The Boolean() Function

Boolean() converts any value to true or false. It is useful for checking how JavaScript will treat a value in a condition:

console.log(Boolean(10 > 9));
console.log(10 > 9);           // even easier: a comparison already is a boolean
console.log(Boolean("hello"));
console.log(Boolean(0));

// Everything With a "Value" Is True

When JavaScript turns a non-boolean value into a boolean, anything that has a real "value" becomes true. These are called truthy:

100              // true
3.14             // true
-15              // true: any non-zero number, even negative
"Hello"          // true
"false"          // true: it is a non-empty string!
"0"              // true: also a non-empty string
" "              // true: a space is a character
(7 + 1 + 3.14)   // true: the result is 11.14
[]               // true: an empty array
{}               // true: an empty object

The last two surprise people. Arrays and objects are always truthy, no matter what is (or is not) inside them, because they are objects, and every object is truthy.

// Everything Without a "Value" Is False

There are exactly eight falsy values. Memorize this list; everything not on it is truthy.

let x;                       // undefined
console.log(Boolean(x));
console.log(Boolean(10 / "Hallo"));   // NaN
console.log(Boolean(""));

// Booleans as Objects (Don't)

Normally a boolean is a simple primitive value: let x = false;. But JavaScript also lets you create a boolean object with the new keyword. You should never do this, but you should recognize it, because it behaves very strangely:

let x = false;
let y = new Boolean(false);

console.log(typeof x);
console.log(typeof y);
console.log(x == y);
console.log(x === y);

if (y) {
  console.log("This runs, even though y holds false!");
}

// Quick Check

let answer = "false";
if (answer) {
  console.log("yes");
} else {
  console.log("no");
}

// Try It

▶ Open Playground

1. Click Open Playground. Run Boolean() on at least eight different values. Predict each answer before you click Run
2. Confirm that " " (a space) is truthy but "" is falsy
3. Store a comparison in a variable, const canVote = age >= 18;, and use it in an if
4. Create new Boolean(false), put it in an if, and see it run. Then try the same with Boolean(false) (no new)