Promise intro ang creating promises

• Promise
  • A promise is an object representing the eventual completion or failure of an asynchronous operation.
  • A PATTERN for writing async code
  • Resolve and reject

  • / THE CALLBACK VERSION
    const fakeRequestCallback = (url, success, failure) => {
        const delay = Math.floor(Math.random() * 4500) + 500;
        setTimeout(() => {
            if (delay > 4000) {
                failure(‘Connection Timeout :(‘)
            } else {
                success(`Here is your fake data from ${url}`)
            }
        }, delay)
    }
    // THE PROMISE VERSION 
    const fakeRequestPromise = (url) => {
        return new Promise((resolve, reject) => {
            const delay = Math.floor(Math.random() * (4500)) + 500;
            setTimeout(() => {
                if (delay > 4000) {
                    reject(‘Connection Timeout :(‘)
                } else {
                    resolve(`Here is your fake data from ${url}`)
                }
            }, delay)
        })
    }

     

• .then.  .catch

• 
  // fakeRequestPromise(‘yelp.com/api/coffee/page1‘)
  //     .then(() => {
  //         console.log("IT WORKED!!!!!! (page1)")
  //         fakeRequestPromise(‘yelp.com/api/coffee/page2‘)
  //             .then(() => {
  //                 console.log("IT WORKED!!!!!! (page2)")
  //                 fakeRequestPromise(‘yelp.com/api/coffee/page3‘)
  //                     .then(() => {
  //                         console.log("IT WORKED!!!!!! (page3)")
  //                     })
  //                     .catch(() => {
  //                         console.log("OH NO, ERROR!!! (page3)")
  //                     })
  //             })
  //             .catch(() => {
  //                 console.log("OH NO, ERROR!!! (page2)")
  //             })
  //     })
  //     .catch(() => {
  //         console.log("OH NO, ERROR!!! (page1)")
  //     })
  
  
  // THE CLEANEST OPTION WITH THEN/CATCH
  // RETURN A PROMISE FROM .THEN() CALLBACK SO WE CAN CHAIN!
  fakeRequestPromise(‘yelp.com/api/coffee/page1‘)
      .then((data) => {
          console.log("IT WORKED!!!!!! (page1)")
          console.log(data)
          return fakeRequestPromise(‘yelp.com/api/coffee/page2‘)
      })
      .then((data) => {
          console.log("IT WORKED!!!!!! (page2)")
          console.log(data)
          return fakeRequestPromise(‘yelp.com/api/coffee/page3‘)
      })
      .then((data) => {
          console.log("IT WORKED!!!!!! (page3)")
          console.log(data)
      })
      .catch((err) => {
          console.log("OH NO, A REQUEST FAILED!!!")
          console.log(err)
      })

   

•  

  creating promises

• const fakeRequest = (url) => {
      return new Promise((resolve, reject) => {
          const rand = Math.random();
          setTimeout(() => {
              if (rand < 0.7) {
                  resolve(‘YOUR FAKE DATA HERE‘);
              }
              reject(‘Request Error!‘);
          }, 1000)
      })
  }
  
  fakeRequest(‘/dogs/1‘)
      .then((data) => {
          console.log("DONE WITH REQUEST!")
          console.log(‘data is:‘, data)
      })
      .catch((err) => {
          console.log("OH NO!", err)
      })

   

• using promise to creat rainbow change color

• const delayedColorChange = (color, delay) => {
      return new Promise((resolve, reject) => {
          setTimeout(() => {
              document.body.style.backgroundColor = color;
              resolve();
          }, delay)
      })
  }
  
  
  delayedColorChange(‘red‘, 1000)
      .then(() => delayedColorChange(‘orange‘, 1000))
      .then(() => delayedColorChange(‘yellow‘, 1000))
      .then(() => delayedColorChange(‘green‘, 1000))
      .then(() => delayedColorChange(‘blue‘, 1000))
      .then(() => delayedColorChange(‘indigo‘, 1000))
      .then(() => delayedColorChange(‘violet‘, 1000))

   

   

Promise intro ang creating promises

原文：https://www.cnblogs.com/LilyLiya/p/14329815.html