JS: Sparse Array

By Xah Lee. Date: 2015-10-31. Last updated: 2023-10-21.

What is a sparse array?

When a array's numerical property key does not exist for index in the range from 0 to n (where n+1 is the value of the property length. 〔see Array.prototype.length〕 )

What is the value of missing index in a sparse array?

It doesn't exist, therefore has no value.

Note, it is different from having values of undefined .

If you access non-existent index, JavaScript return undefined .

🛑 WARNING: Array.prototype.map and others skip missing indexes.

How to Check Missing Keys?

Use Object.keys to see a list of existing indexes.
Use Object.prototype.hasOwnProperty to check a specific index.

// use Object.keys to see existing indexes
const xx = [3, 4, 5];
delete xx[1];
console.log(Object.keys(xx)); // [ "0", "2" ]

// use hasOwnProperty to check if a index exist
const xx = [3, 4, 5];
delete xx[1];
console.log(xx.hasOwnProperty("1") === false);

How to create a sparse array?

Sparse array is created when you call Array(n), where n is a positive integer. 〔see Array Constructor〕
Array becomes sparse when you use “delete” Operator to delete a element.
Array becomes sparse when you set array property length to a number bigger than the index of last item.
Array becomes sparse when a new element is added with index greater than 1 from the last item's index. e.g. const x=["a"]; x[9]="z";

// using delete operator results a sparse array

const xx = ["a", "b"];
delete xx[0];
console.log(xx.length === 2);
console.log(xx.hasOwnProperty("0") === false);

// Example of a sparse array, by adding a item with index greater than length
const xx = ["a", "b"];

console.log(xx.length === 2);

xx[9] = "xyz";
// now xx is sparse array

console.log(xx.length === 10);
console.log(xx.hasOwnProperty("3") === false);

What is the use of sparse array?