使用 JS 是否可以检测 new Image().src 何时将其引用的图像加载到 memory 中？

Question

Background: I'm trying to get the dimensions of an image.

This block successfully creates a new image and assigns it a graphic. Unfortunately this doesn't happen instantly. When I attempt to get height with naturalHeight it returns 0.

let myImage = new Image();
myImage.src = 'images/example.png';

let dimension = myImage.naturalHeight;
alert(dimension); // 0

However if I delay this action with setTimeout() it gives the browser time to load it into memory and it returns the height.

setTimeout(function() {
    let dimension = myImage.naturalHeight;
    alert(dimension); // 479
}, 1000);

Question: Is there a way to detect if the image reference has been fully loaded into memory? This way I can grab naturalHeight after I know the referenced image is loaded.

Answer 1

You can listen to the onload event for the element.

let myImage = new Image();

// Be sure to add the event listener before
// changing the image's source attribute.
myImage.onload = function() {
  let dimension = myImage.naturalHeight;
  alert(dimension);
};

myImage.src = 'images/example.png';

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