function updateRecords(records, id, prop, value) {
if (value === '') {
delete records[id][prop];
} else if (prop !== 'tracks') {
records[id][prop] = value;
} else {
records[id][prop] = records[id][prop] || []; // <= Short-circuit evaluation
records[id][prop].push(value);
}
return records;
}
短絡評価(Short-circuit evaluation)を利用すれば、下のif文が1行で書ける。
if (!records[id][prop]) {
records[id][prop] = [];
}
records[id][prop].push(value);