Promise is returning [object Promise]

问题:

I am trying to do a basic check to see if an image is retina and has a portrait ratio. I have all that checking functionality working perfectly fine. My issue is that in the final function that is supposed to return the boolean I get from my checks it is returning [object Promise]. I have no idea why this is not returning my boolean when I resolve the promise.

When I run .then(res => console.log(res)) it outputs my boolean response, but the function getImageMeta() that returns the promise just returns that [object Promise] which makes me think the promise isn't actually being resolved.

If I could get some assistance that would be great!

/************************************
   Check if image is retina portrait
************************************/
const checkIfPortrait = src => !isRetina(src) ? false : getImageMeta(src).then(res => res);
 const isRetina        = src => src.indexOf('@2x') !== -1;
 const isPortrait      = ({ naturalWidth, naturalHeight }) => naturalHeight > naturalWidth ? true : false;

 function getImageMeta (src) {
    const img = new Image();
    img.src = src;

    return new Promise((resolve, reject) => {
      return img.addEventListener("load", function () {
         return resolve(isPortrait(this));
      }, false);
    });
  }


 export { checkIfPortrait }

This is how I am calling the function:

<img src={media[i].image} alt={media[i].alt} data-portrait={checkIfPortrait(media[i].image)} />

回答1:

This is the expected result. When you return a new Promise object, it will always be a promise object ([object Promise]).

You access the result of a promise by using the .then method (or using await in an async function). Your .then callback is then called when/if the result is made available, which will happen after you call resolve, or if the promise was already resolved prior it will be called rather quickly.

回答2:

The basic idea of a promise is that it gives you something to return right away even if it won't resolve until later. It represents its eventual value. So getImageMeta () is immediately returning the promise even though it has not resolved.

Once you have the promise returned by getImageMeta () you can use its then() to wait for the final boolean, which will come on the next event loop cycle at the earliest.

With your current code you are saving the promise returned by getImageMeta() to checkIfPortrait. So now checkIfPortrait holds this promise and you can call then() on it. Since you are exporting checkIfPortrait you will probably end up calling then on the module that imports it:

 // import checkIfPortrait
checkIfPortrait(src)
.then(ret_val => {
   //you can access boolean here in ret_val
})

回答3:

The issue is now resolved, thanks everyone for the help. Like stated the issue was I was calling the promise's value before it could be resolved and had no way of updating that value after it had been resolved.

So what I did is made sure the image data was accessable before running the function. Then once I had a resolved promise it then update the image data to show whether it is portrait or not.

So final code looks like this:

const enhance = lifecycle({
   componentWillMount: function () {
     Object.keys(this.props.media).map((i) =>
       checkIfPortrait(this.props.media[i].image)
         ? this.props.media[i].isPortrait = true
         : this.props.media[i].isPortrait = false
       );
   }
 })

---

const checkIfPortrait = src => !isRetina(src) ? false : getImageMeta(src).then(res => res);
 const isRetina        = src => src.indexOf('@2x') !== -1;
 const isPortrait      = ({ naturalWidth, naturalHeight }) => naturalHeight > naturalWidth ? true : false;

 function getImageMeta (src) {
   const img = new Image();
   img.src = src;

   return new Promise((resolve, reject) => {
     return img.addEventListener("load", function () {
       return resolve(isPortrait(this));
     }, false);
   });
 }


 export { checkIfPortrait }

---

<img src={media[i].image} alt={media[i].alt} data-portrait={media[i].isPortrait} />