JavaScript写promise

we'll look at some simple examples that you'll encounter in Web APIs.

we'll look at how to write your own promise.


we get you building your own promise-based code.

As we said before, this object represents an intermediate state that is initially neither success nor failure — the official term for a promise in this state is pending.

in promise-speak, when it has been fulfilled (v 实现）


response => response.blob()
function(response){
  return response.blob();
}


let promise2 = promise.then(response => {
  if (!response.ok) {
    throw new Error(`HTTP error! status: ${response.status}`);
  } else {
    return response.blob();
  }
});

let promise3 = promise2.then(myBlob => {


let objectURL = URL.createObjectURL(myBlob);
let image = document.createElement('img');
image.src = objectURL;
document.body.appendChild(image);



});


Something is missing — currently, there is nothing to explicitly handle errors if one of the promises fails (rejects, in promise-speak). 


fetch('coffee.jpg')
.then(response => {
  if (!response.ok) {
    throw new Error(`HTTP error! status: ${response.status}`);
  } else {
    return response.blob();
  }
})
.then(myBlob => {
  let objectURL = URL.createObjectURL(myBlob);
  let image = document.createElement('img');
  image.src = objectURL;
  document.body.appendChild(image);
})
.catch(e => {
  console.log('There has been a problem with your fetch operation: ' + e.message);
});

Bear in mind that the value returned by a fulfilled promise becomes the parameter passed to the next .then() block's callback function.

There was a lot to cover in the above section, so let's go back over it quickly to give you a short guide that you can bookmark and use to refresh your memory in the future. You should also go over the above section again a few more times to make sure these concepts stick.

  1. When a promise is created, it is neither in a success or failure state. It is said to be pending.
  2. When a promise returns, it is said to be resolved.（解决）
    1. A successfully resolved promise is said to be fulfilled. It returns a value, which can be accessed by chaining a .then() block onto the end of the promise chain. The callback function inside the .then() block will contain the promise's return value.
    2. An unsuccessful resolved promise is said to be rejected. It returns a reason, an error message stating why the promise was rejected. This reason can be accessed by chaining a .catch() block onto the end of the promise chain.



let a = fetch(url1);
let b = fetch(url2);
let c = fetch(url3);

Promise.all([a, b, c]).then(values => {
  ...
});


function fetchAndDecode(url, type) {
  return fetch(url).then(response => {
    if(!response.ok) {
      throw new Error(`HTTP error! status: ${response.status}`);
    } else {
      if(type === 'blob') {
        return response.blob();
      } else if(type === 'text') {
        return response.text();
      }
    }
  })
  .catch(e => {
    console.log(`There has been a problem with your fetch operation for resource "${url}": ` + e.message);
  });
}



let coffee = fetchAndDecode('coffee.jpg', 'blob');
let tea = fetchAndDecode('tea.jpg', 'blob');
let description = fetchAndDecode('description.txt', 'text');




Promise.all([coffee, tea, description]).then(values => {

});



console.log(values);
// Store each value returned from the promises in separate variables; create object URLs from the blobs
let objectURL1 = URL.createObjectURL(values[0]);
let objectURL2 = URL.createObjectURL(values[1]);
let descText = values[2];

// Display the images in <img> elements
let image1 = document.createElement('img');
let image2 = document.createElement('img');
image1.src = objectURL1;
image2.src = objectURL2;
document.body.appendChild(image1);
document.body.appendChild(image2);

// Display the text in a paragraph
let para = document.createElement('p');
para.textContent = descText;
document.body.appendChild(para);


se using response.headers.get("content-type"), and then react accordingly.


myPromise
.then(response => {
  doSomething(response);
})
.catch(e => {
  returnError(e);
})
.finally(() => {
  runFinalCode();
});

we chain a finally() call on to the end of the chain
Note:
then()/catch()/finally() is the async equivalent to try/catch/finally in sync code.



function fetchAndDecode(url, type) {
  return fetch(url).then(response => {
    if(!response.ok) {
      throw new Error(`HTTP error! status: ${response.status}`);
    } else {
      if(type === 'blob') {
        return response.blob();
      } else if(type === 'text') {
        return response.text();
      }
    }
  })
  .catch(e => {
    console.log(`There has been a problem with your fetch operation for resource "${url}": ` + e.message);
  })
  .finally(() => {
    console.log(`fetch attempt for "${url}" finished.`);
  });
}
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