Array element is null when I try to get with index

I have a function that get an image size:

  getImageSize(fileUploaded: File): Number[]{

    let size : Number[] = [];
    let _URL = window.URL || window.webkitURL;
    let file, img;
    file = fileUploaded;
    img = new Image();
    img.onload = function() {
      size.push(Number(this.width));
      size.push(Number(this.height));
    };
    img.onerror = function() {
      //alert( "not a valid file: " + file.type);
    };
    img.src = _URL.createObjectURL(file);

    return size;
  }

I use it in another service:

  let iconSize = this.utilService.getImageSize(this.icon[0]) ;
  console.log(iconSize);
  console.log(iconSize.length);
  console.log(iconSize[0]);
  if(Number(iconSize[0]) !== 512 || Number(iconSize[1]) !== 512) {
    this.notifications.create(
      'Error',
      'Icon resolution must be 512*512',
      NotificationType.Error,
      { theClass: 'outline primary', timeOut: 6000, showProgressBar: false }
    );
    return;
  }

The 3 console log returns first the array, but the length is 0 and when I want to get an element it is undefined:

Actually getImageSize is async function. You should promisify it first. Like this

  getImageSize(fileUploaded: File) {
    return new Promise((resolve, reject) => {
      const size: number[] = [];
      const _URL = window.URL || window.webkitURL;
      let file, img;
      file = fileUploaded;
      img = new Image();
      img.onload = function() {
        size.push(Number(this.width));
        size.push(Number(this.height));

        resolve(size);
      };
      img.onerror = function() {
        reject('not a valid file: ' + file.type);
      };
      img.src = _URL.createObjectURL(file);
    });
  }

And then use it in another service with await

let iconSize = await this.utilService.getImageSize(this.icon[0]) ;