Run results for: Every time parse date or second loop with date parse

Run details:

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:121.0) Gecko/20100101 Firefox/121.0

Browser: Firefox 121

Operating system: Windows

Device Platform: Desktop

Date tested: one year ago

Test name	Executions per second
Every time date parse	24515.7 Ops/sec
Parse in second loop	51158.6 Ops/sec

Script Preparation code:

var data = []
var out

for (let i = 0; i < 100; i++)
  data.push({ date: new Date(Math.round(Date.now() * Math.random())).toISOString(), id: Math.floor(i / 25) })
for (let i = 0; i < 100; i++)
  data.push({ date: new Date(Math.round(Date.now() * Math.random())).toISOString(), id: Math.round(Math.random() * 5) })

Tests:

Every time date parse
out = [] for (let i = 0; i < data.length; i++) { const last = out[out.length - 1] const { id, date } = data[i] if (last && last.id === id) last.date = new Date(date) else out.push({ id, date: new Date(date) }) }
Parse in second loop
out = [] for (let i = 0; i < data.length; i++) { const last = out[out.length - 1] const { id, date } = data[i] if (last && last.id === id) last.date = date else out.push({ id, date: date }) } for (let i = 0; i < out.length; i++) out[i].date = new Date(out[i].date)

Run results for: Every time parse date or second loop with date parse

Every time date parse

Parse in second loop