Array
Asynchronous
Data Type
Declarations
Empty Slot
Function
Guidance
IO
Keyed Collections
Loop
Math
Modules
Object
String
Symbol
Template Literal
Web Worker
Propertyempty slotundefined
How it is createdCreated by sparse array syntax such as new Array(5)Explicit assignment like arr[0] = undefined
Whether a value existsNo value is stored at allA value exists, and that value is undefined
Access resultarr[i] // undefined because the slot does not existarr[i] // undefined
in operatori in arr // falsei in arr // true
hasOwnPropertyarr.hasOwnProperty(i) // falsearr.hasOwnProperty(i) // true
JSON.stringifyEmpty slots may be skippedThe position is preserved as a value
map / forEach iterationThat index is skippedThat index is iterated normally
Example difference[ , , 3] -> length = 3, but the first two positions are empty slots[undefined, undefined, 3] -> length = 3, and the first two positions do contain values

Example

const arr1 = new Array(3); // [ <3 empty items> ]
const arr2 = [undefined, undefined, undefined];


console.log(0 in arr1); // false  -> the slot does not exist
console.log(0 in arr2); // true   -> the value exists and is undefined


console.log(arr1.hasOwnProperty(0)); // false
console.log(arr2.hasOwnProperty(0)); // true


arr1.forEach((x, i) => console.log(i, x)); // prints nothing
arr2.forEach((x, i) => console.log(i, x)); // prints 0 undefined, 1 undefined, 2 undefined

Quick memory trick:

  • empty slot: no stored value, the position is absent
  • undefined: a stored value exists, but that value is empty