Default and Rest Parameters

In short: function f(x = 50) {} gives x a default value — but that default only kicks in when the caller passes undefined or leaves the argument out entirely; an explicit 0, "", or false is a real value and is used as-is. function f(first, ...rest) {} uses the rest parameter to scoop up every argument after the named ones into a genuine array — it must be the last parameter, since nothing could come after it.

Fall back to a default volume

A default only fills in when the argument is genuinely missing — undefined, whether from an omitted argument or an explicit one:

play(80) prints 80 — the caller supplied a value, so the default is never consulted. play() prints 50 — no argument at all means volume is undefined, which triggers the default. play(0) prints 0, not 50 — 0 is an explicit, real argument, and defaults only ever replace undefined, exactly the same rule ??= uses for variables.

Gather every extra song into a real array

...rest collects whatever arguments are left after the named parameters, packaged as an actual array with every array method available:

first takes the first argument, "Blue Hour". rest, being the final parameter, absorbs everything after it — ["Red Dawn", "Green Tide"] — as a real array, so rest.length, rest.map(...) and so on all work directly, unlike the older arguments object.

Quick practice

🎯 Your turn

Explain what function play(volume = 50) { console.log(volume); } play(0); prints and why, and what function queue(first, ...rest) { console.log(rest); } queue("A", "B", "C"); prints and why.

Write your explanation and submit it — there's no code to run here, just your reasoning.