I want to test the performance of some code snippets in JS using:

function perf() {
  var start = new Date()
  // the snippet codes
  var end = new Date()
  return end - start

then i wrote some sample code:


    <button>insert list item</button>


function insertListItem() {
  let ul = document.querySelector('ul')
  let start = new Date()

  // heavy dom operations
  for (let i = 0; i < N; i++) {
    let li = document.createElement('li')
    li.textContent = 'item' + i

  // log the duration (deferred by 0ms timer)
  setTimeout(() => {
    let t2 = new Date() - start
    console.log(`t2: ${t2} ms`)
  }, 0)

  // log the duration instantly
  let t1 = new Date() - start
  console.log(`t1: ${t1} ms`)

let N = 100000
let btn = document.querySelector('button')
btn.addEventListener('click', insertListItem)

The console output was: t1: 199 ms t2: 7332 ms

It looks like t2 is the performance i want, but wrap them with a deferred timer is weird, what is right way to do this? And why is t1 so fast?


1 Answer 1


Use Chrome's Timeline tool to measure performance.

  • Make a Timeline recording to analyze every event that occurred after a page load or a user interaction.

  • View FPS, CPU, and network requests in the Overview pane.

  • Click on an event within the Flame Chart to view details about it.

  • Zoom in on a section of a recording to make analysis easier.

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