JavaScript Booleans

Two literals

A boolean is true or false. Write them in lowercase. True andFalse are Python; in JavaScript they are ordinary names, not the boolean values.

You store a yes-or-no fact: a box is checked, a score is high enough, a list still has items. Conditions consume booleans. Comparisons produce them.

Comparisons produce booleans

7 > 3 is true. 7 === 3 is false. You can store that result, log it, or pass it into if in the next chapter. The comparison operators are covered in full under Comparisons.

const ready = true;
const empty = false;
console.log(ready);
console.log(empty);
console.log(7 > 3);
console.log(7 === 3);
console.log(typeof ready);

document.body.textContent =
  String(ready) + " " + String(7 > 3) + " " + typeof ready;

Boolean(value)

Boolean(value) converts any value to true or false using the same rule if uses. It does not change the original value. Call it when you want to seehow a value will behave as a condition.

console.log(Boolean(1));
console.log(Boolean(0));
console.log(Boolean("hi"));
console.log(Boolean(""));
console.log(Boolean(null));
console.log(Boolean([]));

document.body.textContent =
  String(Boolean("hi")) + " " + String(Boolean("")) + " " + String(Boolean(0));

Truthy and falsy

Values that become false in a boolean context are called falsy. Everything else is truthy. Memorize the falsy list. It is short.

"0" is a non-empty string, so it is truthy. [] and {} are objects, so they are truthy even when they hold nothing.

if (value) uses the same rule

You do not have to write if (Boolean(value) === true). if (value) already converts. An empty string skips the block. A non-empty string runs it. 0 skips. Any other number runs.

function label(value) {
  if (value) {
    return "truthy";
  }
  return "falsy";
}

console.log(label("hello"));
console.log(label(""));
console.log(label(0));
console.log(label(3));
console.log(label(undefined));

document.body.textContent =
  label("hello") + " / " + label("") + " / " + label(0);

Not, and double not

!value flips a boolean: !true is false. Applied to a non-boolean, it converts first, then flips: !"" is true because the empty string is falsy.

!!value converts to boolean twice and lands on the same result as Boolean(value). You will see it in other people’s code. Boolean(value) is easier to read.

What to remember

• Literals are true and false in lowercase.
• Comparisons produce booleans.
• The falsy list is short. Empty array and empty object are truthy.
• Boolean(value) and if (value) follow that same list.

Next: comparisons — === versus ==, and why the extra equals sign matters.