HTML5 Video Dimensions

It should be noted that the currently accepted solution above by Sime Vidas doesn't actually work in modern browsers since the videoWidth and videoHeight properties aren't set until after the "loadedmetadata" event has fired.

If you happen to query for those properties far enough after the VIDEO element is rendered it may sometimes work, but in most cases this will return values of 0 for both properties.

To guarantee that you're getting the correct property values you need to do something along the lines of:

var v = document.getElementById("myVideo");
v.addEventListener( "loadedmetadata", function (e) {
    var width = this.videoWidth,
        height = this.videoHeight;
}, false );

NOTE: I didn't bother accounting for pre-9 versions of Internet Explorer which use attachEvent instead of addEventListener since pre-9 versions of that browser don't support HTML5 video, anyway.

<video id="foo" src="foo.mp4"></video>

var vid = document.getElementById("foo");
vid.videoHeight; // returns the intrinsic height of the video
vid.videoWidth; // returns the intrinsic width of the video

Spec: https://html.spec.whatwg.org/multipage/embedded-content.html#the-video-element

Ready-to-use function

Here's a ready to use function which returns the dimensions of a video asynchrously, without changing anything in the document.

// ---- Definitions ----- //

/**
 Returns the dimensions of a video asynchrounsly.
 @param {String} url Url of the video to get dimensions from.
 @return {Promise<{width: number, height: number}>} Promise which returns the dimensions of the video in 'width' and 'height' properties.
 */
function getVideoDimensionsOf(url){
    return new Promise(resolve => {
        // create the video element
        const video = document.createElement('video');

        // place a listener on it
        video.addEventListener( "loadedmetadata", () => {
            // retrieve dimensions
            const height = this.videoHeight;
            const width = this.videoWidth;

            // send back result
            resolve({height, width});
        }, false);

        // start download meta-datas
        video.src = url;
    });
}


// ---- Use ---- //
getVideoDimensionsOf("https://www.w3schools.com/html/mov_bbb.mp4")
   .then(console.log);

Here's the video used for the snippet if you wish to see it : Big Buck Bunny