Get width height of remote image from url

Get image size with jQuery

function getMeta(url) {
    $("<img/>",{
        load: function() {
            alert( this.width +" "+ this.height );
        },
        src: url
    });
}

Get image size with JavaScript

function getMeta(url) {   
    var img = new Image();
    img.onload = function() {
        alert( this.width +" "+ this.height );
    };
    img.src = url;
}

Get image size with JavaScript (modern browsers, IE9+ )

function getMeta(url){   
    const img = new Image();
    img.addEventListener("load", function() {
        alert( this.naturalWidth +' '+ this.naturalHeight );
    });
    img.src = url;
}

Use like: getMeta( "http://example.com/img.jpg" );

https://developer.mozilla.org/en/docs/Web/API/HTMLImageElement

Just pass a callback as argument like this:

function getMeta(url, callback) {
    const img = new Image();
    img.src = url;
    img.onload = function() { callback(this.width, this.height); }
}
getMeta(
  "http://snook.ca/files/mootools_83_snookca.png",
  (width, height) => { alert(width + 'px ' + height + 'px') }
);

The w and h variables in img.onload function are not in the same scope with those in the getMeta() function. One way to do it, is as follows:

Fiddle: http://jsfiddle.net/ppanagi/28UES/2/

function getMeta(varA, varB) {
    if (typeof varB !== 'undefined') {
       alert(varA + ' width ' + varB + ' height');
    } else {
       var img = new Image();
       img.src = varA;
       img.onload = getMeta(this.width, this.height);
    }
}


getMeta("http://snook.ca/files/mootools_83_snookca.png");

ES6

Using async/await you can do below getMeta function in sequence-like way and you can use it as follows (which is almost identical to code in your question (I add await keyword and change variable end to img, and change var to let keyword). You need to run getMeta by await only from async function (run).

function getMeta(url) {
    return new Promise((resolve, reject) => {
        let img = new Image();
        img.onload = () => resolve(img);
        img.onerror = () => reject();
        img.src = url;
    });
}

async function run() {

  let img = await getMeta("http://shijitht.files.wordpress.com/2010/08/github.png");

  let w = img.width;
  let h = img.height; 

  size.innerText = `width=${w}px, height=${h}px`;
  size.appendChild(img);
}

run();

<div id="size" />

Rxjs

const { race, fromEvent, map, mergeMap, of } = rxjs;

function getMeta(url) {
    return of(url).pipe(
        mergeMap((path) => {
            const img = new Image();
            let load = fromEvent(img, 'load').pipe(map(_=> img))
            let error = fromEvent(img, 'error').pipe(mergeMap((err) => throwError(() => err)));
            img.src = path;
            
            return race(load, error);
        })
    );
}


let url = "http://shijitht.files.wordpress.com/2010/08/github.png";

getMeta(url).subscribe(img=> { 
  let w = img.width;
  let h = img.height; 

  size.innerText = `width=${w}px, height=${h}px`;
  size.appendChild(img);
}, e=> console.log('Load error'))

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/7.5.2/rxjs.umd.min.js" integrity="sha512-wBEi/LQM8Pi08xK2jwHJNCiHchHnfcJao0XVQvkTGc91Q/yvC/6q0xPf+qQr54SlG8yRbRCA8QDYrA98+0H+hg==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<div id="size" />