Setting the file. One moment.
Skill 03 · Vercel React Best Practices
Subchapter 3.35
rules/js-length-check-first.mdMarkdown2 KBView on GitHub
When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.
In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).
Incorrect (always runs expensive comparison):
function hasChanges(current: string[], original: string[]) {
// Always sorts and joins, even when lengths differ
return current.sort().join() !== original.sort().join()
}Two O(n log n) sorts run even when current.length is 5 and original.length is 100. There is also overhead of joining the arrays and comparing the strings.
Correct (O(1) length check first):
function hasChanges(current: string[], original: string[]) {
// Early return if lengths differ
if (current.length !== original.length) {
return true
}
// Only sort when lengths match
const currentSorted = current.toSorted()
const originalSorted = original.toSorted()
for (let i = 0; i < currentSorted.length; i++) {
if (currentSorted[i] !== originalSorted[i]) {
return true
}
}
return false
}This new approach is more efficient because: