JavaScript: Array.prototype.length

By Xah Lee. Date: . Last updated: .
arrayX.length
The value is number of elements of array.
console.log([3, 4].length === 2);

Array length can be set. If you set it, the array will be lengthened (becomes Sparse Array) or shortened (tail elements removed).

Length is Own Property

Each array has its own property "length".

console.log([3, 4].hasOwnProperty("length"));

Length Property Cannot be Deleted

Each array's own property "length" has Property Attribute configurable false, so you cannot delete it.

// the length property of array has configurable attribute false
console.log(
  Object.getOwnPropertyDescriptor(
    [3, 4],
    "length",
  ).configurable === false,
);
// indexes of array are enumerable
console.log(
  Object.getOwnPropertyDescriptor(
    [3, 4],
    "0",
  ).enumerable === true,
);
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