Update description of Array.length property (#43981)

* Update description of Array.length property

Clarified the definition of the 'length' property to indicate it represents the number of slots in the array, accounting for sparse arrays.

* Update files/en-us/web/javascript/reference/global_objects/array/length/index.md

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

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Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

Author: jonrandy

files/en-us/web/javascript/reference/global_objects/array/length/index.md

@@ -7,7 +7,7 @@ browser-compat: javascript.builtins.Array.length
 sidebar: jsref
 ---
 
-The **`length`** data property of an {{jsxref("Array")}} instance represents the number of elements in that array. The value is an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.
+The **`length`** data property of an {{jsxref("Array")}} instance represents the number of slots in that array. The value is an unsigned, 32-bit integer that is always numerically greater than the highest index in the array. It may be greater than the number of elements if the array is [sparse](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#array_methods_and_empty_slots).
 
 {{InteractiveExample("JavaScript Demo: Array: length", "shorter")}}