Truthy and Falsy

Every value answers yes or no

Anywhere a condition is expected, JavaScript converts whatever it is given to a boolean. Most values are truthy; the falsy ones are a short list worth learning by heart.

const falsy = [false, 0, -0, 0n, '', null, undefined, NaN];

console.log(falsy.map(Boolean));

[false, false, false, false, false, false, false, false]

That is the complete list: false, 0, -0, 0n, the empty string, null, undefined and NaN. Everything else is truthy.

The ones that surprise people

console.log(Boolean('0'));
console.log(Boolean('false'));
console.log(Boolean([]));
console.log(Boolean({}));
console.log(Boolean(-1));

true
true
true
true
true

An empty array and an empty object are both truthy, because they are objects and every object is truthy. Any non-empty string is truthy, including "0" and "false".

Where it bites

function describe(count) {
  if (!count) {
    return 'nothing to show';
  }
  return count + ' items';
}

console.log(describe(5));
console.log(describe(0));

5 items
nothing to show

Checking emptiness

Because [] and {} are truthy, emptiness has to be asked about directly.

const list = [];
const settings = {};

console.log(list.length === 0);
console.log(Object.keys(settings).length === 0);

true
true

Converting on purpose

Boolean(value) is the clear way to convert. You will also see !!value, which is two negations and does the same thing.

const name = 'Ada';

console.log(Boolean(name), !!name);

true true