description: Imported skill js_combine_iterations from vercel name: js_combine_iterations signature: 71add08aeeb43091d4ff4c0b2842cce8b4bdef8ad3e732cc034bb5a84827e746 source: /a0/tmp/skills_research/vercel/skills/react-best-practices/rules/js-combine-iterations.md

title: Combine Multiple Array Iterations impact: LOW-MEDIUM impactDescription: reduces iterations tags: javascript, arrays, loops, performance

Combine Multiple Array Iterations

Multiple .filter() or .map() calls iterate the array multiple times. Combine into one loop.

Incorrect (3 iterations):

typescript

const admins = users.filter(u => u.isAdmin)
const testers = users.filter(u => u.isTester)
const inactive = users.filter(u => !u.isActive)

Correct (1 iteration):

typescript

const admins: User[] = []
const testers: User[] = []
const inactive: User[] = []

for (const user of users) {
  if (user.isAdmin) admins.push(user)
  if (user.isTester) testers.push(user)
  if (!user.isActive) inactive.push(user)
}