undefined and null

In short: JavaScript has two different "empty" values, and they mean different things. undefined is what the engine gives you when something was never set — a missing property, an unassigned variable, a function that falls off the end without a return. null is what a person writes on purpose to say "this is deliberately empty." Mixing the two up leads to checks that look right but miss half the cases.

A song with no album yet

A song object can have a field a person filled in as empty, and a field nobody ever added at all — they read very differently:

song.album is null: someone decided this song has no album and recorded that fact. song.year is undefined: the property was never added, so the engine has nothing to report. Same "empty" feeling, opposite origin.

A function that forgets to return

Every function returns something, even when its body doesn't say so:

logPlay never has a return, so calling it gives undefined back — the engine's default answer when you ask a function for a value it didn't promise one. This is the single most common way undefined shows up by accident: a forgotten return.

Reading a property of nothing

The real danger isn't the empty value itself — it's reaching into one:

song.year is undefined, and undefined has no properties, so .length throws a TypeError. The fix is to check first: song.year ? song.year.length : 0, or the shorter song.year?.length, which returns undefined instead of throwing when the left side is missing.

One-liners worth knowing: null == undefined is true (loose equality treats them as the same kind of empty), but null === undefined is false (different types). typeof null is "object" — a decades-old leftover bug kept for compatibility — while typeof undefined is "undefined".

Quick practice