async await in image loading

Your problem here extends from the definition for await…

The await operator is used to wait for a Promise

The Image.prototype.onload property is not a promise, nor are you assigning it one. If you’re wanting to return the height property after loading, I would instead create a Promise…

addImageProcess(src){
  return new Promise((resolve, reject) => {
    let img = new Image()
    img.onload = () => resolve(img.height)
    img.onerror = reject
    img.src = src
  })
}

You would then use the following to access that value

tmp.addImageProcess(imageUrl).then(height => {
  console.log(height)
})

or, if within an async function

async function logImageHeight(imageUrl) {
  console.log('height', await tmp.addImageProcess(imageUrl))
}