test-splice.js

const x = [
  'header-c', 
  'value',
  'header-a',
  'value',
  'header-b',
  'value',
  'header-e', 
  'value',
  'header-d',
  'value',
  'header-f',
  'value'
]

function sort1 (headers) {
  const names = []
  for (let i = 0; i < headers.length; i += 2) {
    names.push(headers[i])
  }
  names.sort()
  const sorted = []
  for (const name of names) {
    sorted.push(name)
    sorted.push(headers[headers.indexOf(name) + 1])
  }
  return sorted
}

function sort2 (headers) {
  let i = headers.length
  let previous
  while (i -= 2) {
    if (headers[i - 2] > headers[i]) {
      headers.push(...headers.splice(i - 2, 2))
    }
  }
}

const funcs = { sort1, sort2 }

for (let i = 0; i < 1000000; i++) {
  funcs[process.argv[2]](x)
}