Will a browser give an iframe a separate thread for JavaScript?
Recently tested if JavaScript running in a iFrame would block JavaScript from running in the parent window.
iFrame on same domain as parent:
- Chrome 68.0.3440.84: Blocks
- Safari 11.0.2 (13604.4.7.1.3): Blocks
- Safari 15.1 on iOS: Blocks
- Firefox 96: Blocks
iFrame on different domain as parent
- Chrome 68.0.3440.84: Doesn't block
- Safari 11.0.2 (13604.4.7.1.3): Blocks (outdated, but I don't have a macbook)
- Safari 15.1 on iOS: Doesn't block
- Firefox 96: Doesn't block
- Chrome for Android 96: sometimes Blocks and sometimes Doesn't block (There are some complex rules in Chrome for Android that determine when Chrome for Android does and doesn't isolate a process, see
chrome://process-internalsandchrome://flags)
parent.html:
<body>
<div id="count"></div>
<iframe src="./spin.html"></iframe>
<script>
let i = 0;
let div = document.getElementById("count");
setInterval(() => {
div.innerText = i++;
}, 100);
</script>
</body>
spin.html:
<body>
<button id="spin">spin</button>
<script>
const spin = document.getElementById("spin");
spin.addEventListener('click', () => {
const start = Date.now();
while (Date.now() - start < 1000) { }
})
</script>
</body>
To sum up the other answers: No, iFrames usually run in the same thread/process as the main page.
However, it appears the Chromium team are working on further isolation in this area:
Chromium Issue 99379: Out of process iframes [sorry, link not working - if you can find a link to the issue that works, please let me know]
Design Plans for Out-of-Process iframes
Before chrome came along, all tabs of any browser shared the same single thread of JavaScript. Chrome upped the game here, and some others have since followed suit.
This is a browser implementation detail, so there is no solid answer. Older browsers definitely don't. I don't know of any browser that definitely uses another thread for iframes, but to be honest I've never really looked into it.
It isn't a security risk, as no objects are brought along with the thread execution.