JavaScript Array.sort() Method - Sorting Arrays Correctly

Understanding Array.sort()

Array.sort() sorts elements in place and returns the sorted array. Without a compare function, elements are converted to strings and sorted by UTF-16 code unit order.

The Default Sort Pitfall

// WRONG — default sorts as strings!
[10, 1, 21, 2].sort();
// Result: [1, 10, 2, 21] (string order)

// CORRECT — use a compare function for numbers
[10, 1, 21, 2].sort((a, b) => a - b);
// Result: [1, 2, 10, 21]

Compare Function Rules

The compare function must return:

• Negative if a should come before b
• Zero if they are equal
• Positive if a should come after b

// Ascending numbers
arr.sort((a, b) => a - b);

// Descending numbers
arr.sort((a, b) => b - a);

// Alphabetical strings (locale-aware)
arr.sort((a, b) => a.localeCompare(b));

// Sort objects by property
users.sort((a, b) => a.name.localeCompare(b.name));

// Sort by date
events.sort((a, b) => new Date(a.date) - new Date(b.date));

Stability

As of ES2019, sort() is guaranteed to be stable — elements that compare equal retain their original relative order.

sort() Mutates!

sort() modifies the original array. If you need to preserve the original:

// Pre-ES2023 workaround
const sorted = [...arr].sort((a, b) => a - b);

// ES2023: toSorted()
const sorted = arr.toSorted((a, b) => a - b);
// arr is unchanged

Performance

sort() uses an implementation-dependent algorithm (usually TimSort in V8/SpiderMonkey), with O(n log n) average-case complexity. For very large arrays (100k+ elements), consider whether you actually need a full sort or just the top N elements.