Array、Stringはlength
Set、Mapはsize で取得できる
// Array.length
[1,2,3,4,5].length // 5
// String.length
'hoge'.length // 4
// Set.proptotype.size
new Set([1,2,3,3,3,3]).size // 3
// Map.prototype.size
new Map([[0, 'a'], [1, 'b']]).size // 2
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