JavaScript Booleans

A boolean has only two possible values: true and false. Every decision in a program ends up as one of them.

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const isLoggedIn = true;
const hasDiscount = false;

console.log(isLoggedIn);   // → true
console.log(5 > 3);        // → true
console.log(5 === 6);      // → false

Comparisons such as >, < and === produce booleans. You will use them in if statements, loops and filters.

Naming booleans

Name them so they read like a question: isOpen, hasItems, canSubmit. Then conditions read like English:

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const cart = ["book", "pen"];
const hasItems = cart.length > 0;

if (hasItems) {
  console.log("Show the checkout button");
}

Converting to boolean

Any value can be turned into a boolean with Boolean() or with a double not !!.

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console.log(Boolean("hello")); // → true
console.log(Boolean(""));      // → false
console.log(Boolean(0));       // → false
console.log(Boolean(42));      // → true
console.log(!!"text");         // → true

JavaScript uses this conversion automatically in conditions. The values that become false are called falsy and the rest are truthy. See Truthy and Falsy for the full list.

A common mistake with the strings “true” and “false”

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console.log(Boolean("false")); // → true

Any non-empty string is truthy, including the text "false". When you read a value from a form or storage, it is a string. Compare it with === "true" instead of converting it directly.