Truthy, Falsy, null, and undefined

JavaScript conditions convert values to booleans. Common falsy values include false, 0, -0, 0n, "", null, undefined, and NaN. Nearly everything else is truthy, including empty arrays and empty objects.

if ([]) console.log("An empty array is truthy");
if ({}) console.log("An empty object is truthy");

null versus undefined#

undefined commonly means a value was not assigned or a property is absent. null is often used to represent an intentional absence. Choose a consistent convention in your application rather than mixing both without a reason.

const profile = { nickname: null };
console.log(profile.email); // undefined: property does not exist
console.log(profile.nickname); // null: property exists with explicit absence

Default values#

Use || when any falsy value should trigger the fallback. Use ?? when only null and undefined should trigger it.

const suppliedLimit = 0;
const limitWithOr = suppliedLimit || 10; // 10
const limitWithNullish = suppliedLimit ?? 10; // 0

Use explicit checks when zero, an empty string, or false are meaningful values. Avoid vague truthiness checks for form values if the valid domain is more specific.

Practice#

Create a function that accepts an optional page size. Verify that 0 is preserved if it is a meaningful input, while null and undefined use a default.