Optional chaining and nullish coalescing

Model answer.

• Optional chaining `?.` stops and returns undefined if the value on its left is null or undefined, instead of throwing. It works for properties, calls and indexes: user?.address?.city, callback?.(), list?.[0].
• Nullish coalescing `??` returns the right side only when the left is null or undefined.

The difference from ||: || falls back for any falsy value, including legitimate ones like 0, '' and false.

const settings = { volume: 0, nickname: '' };
settings.volume || 50;    // 50: wrong, the user chose 0
settings.volume ?? 50;    // 0: correct
settings.nickname ?? 'Guest';  // '': kept, because it was set

Use ?? for defaults when 0, empty string or false are valid values. Use || only when every falsy value should really be replaced. (You cannot mix ?? with ||/&& without brackets; JavaScript forces you to be explicit.)