JavaScript Element transitionstart Event

Beginner
5 min read
Updated: Nov 2026
3 examples
Event
Decorative image with the word TRANSITIONSTART

Definition

In JavaScript, the transitionstart event fires when a CSS transition has actually started—after any transition-delay has ended (MDN). The event object is a TransitionEvent. It is not cancelable. Creation (before delay) is transitionrun.

Register a listener with addEventListener. Check readiness and browser support before you rely on this event.

Syntax

Listen for transitionstart. Start with this line.

JavaScript
el.addEventListener("transitionstart", fn)
CallWhat it does
el.addEventListener("transitionstart", fn)Primary signature.

Built into the browser. Listen with el.addEventListener("transitionstart", …) — no import required.

Library declarations
JavaScript
el.addEventListener("transitionstart", fn)

Parameters

ParameterDescription
Event targetRequired. Element whose animated values just began changing
TransitionEventRequired. Carries propertyName, elapsedTime, pseudoElement
Delay (optional)Optional. Makes the gap after transitionrun visible

Return Value

Return ValueDescription
*Fires as an Element event (listener receives the Event object).

JavaScript Examples

Example 1 — Listen for transitionstart

Log a message when animated values actually begin changing.

JavaScript
const box = document.getElementById("box");
const out = document.getElementById("out");

box.addEventListener("transitionstart", () => {
  out.textContent = "transitionstart fired";
});
Try It Yourself

How It Works

  1. Pick the element. The node that will animate a CSS property.
  2. Register the listener. transitionstart runs when motion begins.
  3. Show feedback. Update status text at the start of the animation.
  4. Trigger in the lab. Change a transitioned style after setting a non-zero duration.

Example 2 — Read TransitionEvent Fields

Print propertyName when the animation begins.

JavaScript
const box = document.getElementById("box");
const out = document.getElementById("out");

box.addEventListener("transitionstart", (event) => {
  out.textContent = "start: " + event.propertyName;
});
Try It Yourself

How It Works

  1. Receive a TransitionEvent. Start uses the same event type as other transition events.
  2. Read the property. propertyName is which CSS property began animating.
  3. Filter when needed. Ignore properties you do not care about.
  4. Use for UI. Toggle “animating” state when motion really starts.

Example 3 — Start vs Run (with Delay)

Log both events so the delay gap is obvious.

JavaScript
const box = document.getElementById("box");
const out = document.getElementById("out");
const lines = [];

function log(msg) {
  lines.push(msg);
  out.textContent = lines.join("\\n");
}

box.addEventListener("transitionrun", () => {
  log("transitionrun");
});

box.addEventListener("transitionstart", () => {
  log("transitionstart");
});
Try It Yourself

How It Works

  1. Listen for both. Run is creation; start is when animation begins.
  2. Use a delay in the lab. For example opacity 0.3s linear 0.5s.
  3. See the order. transitionrun prints first; transitionstart after the delay.
  4. No delay? MDN: both fire (essentially together).
Compatibility

Browser Support

transitionstart is Baseline Widely available on MDN (across browsers since March 2020). It is a standard CSS Transitions event, not Deprecated or Experimental.

✓ Baseline · Widely available

Element transitionstart

Fires when a CSS transition actually starts after any delay. Supported in modern Chrome, Firefox, Safari, and Edge.

95% Modern browser support
Google Chrome Supported · Desktop & Mobile
Full support
Mozilla Firefox Supported · Desktop & Mobile
Full support
Apple Safari Supported · macOS & iOS
Full support
Microsoft Edge Supported · Chromium
Full support
Internet Explorer No native support · Use a polyfill
Polyfill
Opera Supported · Modern versions
Full support
Samsung Internet Supported · Android
Full support
Android WebView Supported · Modern WebView
Full support
transitionstart Excellent

Bottom line: Safe to use in modern web apps. Pair with transitionrun when you need “created” vs “animation began.”

Frequently Asked Questions

It fires when a CSS transition has actually started—after any transition-delay has ended and the property values begin changing (MDN). It is not cancelable.
No. MDN does not mark Element transitionstart as Deprecated, Experimental, or Non-standard. It is Baseline Widely available (across browsers since March 2020).
transitionrun fires when the transition is created (start of any delay). transitionstart fires when the actual animation begins (end of any delay). With no delay or a negative delay, both fire.
A TransitionEvent. Useful read-only properties include propertyName, elapsedTime, and pseudoElement.
If the transition is canceled during the delay (after transitionrun), you may never see transitionstart or transitionend—you get transitioncancel instead.
Prefer element.addEventListener("transitionstart", handler). You can also set ontransitionstart where supported.

Next: webkitmouseforcechanged

Continue with the next Element event in the sidebar.

webkitmouseforcechanged →

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

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