Question

So the alert gives undefined values for the width and height. I think the w and h values of the image from the img.onload calculation is not being passed to the values to return, or it may be returning w and h before the onload calculates them:

function getMeta(url){
 var w; var h;
 var img=new Image;
 img.src=url;
 img.onload=function(){w=this.width; h=this.height;};
 return {w:w,h:h}    
}

// "http://snook.ca/files/mootools_83_snookca.png" //1024x678
// "http://shijitht.files.wordpress.com/2010/08/github.png" //128x128

var end = getMeta("http://shijitht.files.wordpress.com/2010/08/github.png");
var w = end.w;
var h = end.h;
alert(w+'width'+h+'height');

How can I have the alert show the correct width and height?

http://jsfiddle.net/YtqXk/

Answer 1

Get image size with JavaScript

In order to read the data from an image you’ll need to make sure it’s first loaded. Here’s a callback-based approach and two promise-based solutions:

Callback

const getMeta = (url, cb) => {
  const img = new Image();
  img.onload = () => cb(null, img);
  img.onerror = (err) => cb(err);
  img.src = url;
};

// Use like:
getMeta("https://i.stack.imgur.com/qCWYU.jpg", (err, img) => {
  console.log(img.naturalWidth, img.naturalHeight);
});

Using the load Event listener (Promise):

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

// Usage example: 
;(async() => {
  const img = await getMeta('https://i.stack.imgur.com/qCWYU.jpg');
  console.dir(img.naturalHeight + ' ' + img.naturalWidth);
})();

Using HTMLImageElement.decode() (Promise)

const getMeta = async (url) => {
  const img = new Image();
  img.src = url;
  await img.decode();  
  return img
};

// Usage example:
getMeta('https://i.stack.imgur.com/qCWYU.jpg').then(img => {
  console.dir(img.naturalHeight +' '+ img.naturalWidth);
});

• MDN Docs: HTMLImageElement