//配列
const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
console.log(arr.length)  //10

//length プロパティに値を指定して、配列を短縮できます。
arr.length = 12
console.log(arr); // [1,2,3,4,5,6,7,8,9,10,,]

arr.length = 5  // [1,2,3,4,5]
console.log(arr);

//オブジェクト
const obj = {
  a:1,
  b:2,
  c:3,
}
console.log(Object.keys(obj).length)  //3

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