"dense" and "sparse" array

JavaScriptJunior~1 min read

Definitions

Dense array

An array in which all indexes are occupied by elements: from 0 to length - 1.

That is:

there are no "missing" indexes (holes),
every index actually exists on the object.

javascript

const dense = [10, 20, 30];

Here:

indexes: 0, 1, 2
dense.length === 3
0 in dense -> true, 1 in dense -> true, 2 in dense -> true

Sparse array

An array in which some indexes are missing: it has "holes".

javascript

const sparse = [10, , 30];
console.log(sparse); // [10, <1 empty item>, 30]

Here:

index 1 does not exist (1 in sparse -> false)
sparse.length === 3, but there are actually only 2 elements

How "holes" appear

When using delete:

javascript

const arr = [1, 2, 3];
delete arr[1];
console.log(arr); // [1, <1 empty item>, 3]

When specifying a gap in a literal:

javascript

const arr = [1, , 3];

When increasing length manually:

javascript

const arr = [1, 2];
arr.length = 5;
console.log(arr); // [1, 2, <3 empty items>]

Behavior during iteration

Method	Dense array	Sparse array
`for`	goes through all indexes	goes through all indexes
`for...of`	goes through all values	skips empty ones
`forEach()`	goes through every element	skips "empty" ones
`map()`, `filter()`, `reduce()`	skip "empty" ones
`in`	checks for the index	returns `false` for holes
`Object.keys()`	shows only existing indexes	skips empty ones

Comparison example:

javascript

const dense = [1, 2, 3];
const sparse = [1, , 3];

dense.forEach(v => console.log(v));  // 1, 2, 3
sparse.forEach(v => console.log(v)); // 1, 3 (index 1 skipped)

console.log(Object.keys(dense));  // ["0", "1", "2"]
console.log(Object.keys(sparse)); // ["0", "2"]

Impact on performance

Sparse arrays:

run slower,
are optimized worse by the V8 engine,
and often behave "strangely" during iteration.

That is why in real projects it is better to avoid "holey" arrays and use null or undefined as explicit "empty" values.

In short:

Array type	Description	Example	Behavior
Dense	All indexes are occupied	`[1, 2, 3]`	Predictable, fast
Sparse	Has missing indexes	`[1, , 3]`	Methods skip empty ones, slower

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