JS Promise #7

Promise

• Promise is JS Object for asynchronous operations.
• State : pending > fulfilled or rejected
• Producer vs Consumer

Producer

• When new Promise is created, executer runs automatically.

  const promise = new Promise((resolve, reject) =>{
   console.log("doing something....")
   setTimeout(() => {
     resolve('gyus')
     //reject(new Error('no Network'));
     }
   )
  })

• Resolve : 콜백함수가 잘 처리되었을 때

• Reject : 콜백 함수가 잘 못 처리되었을 때

  Consumer

  promise
   .then(value => {
     console.log(value)
   })
   .catch(error => {
     console.log(error);
   })
   .finally( () =>{
     console.log("finally")
   })
  

• then : resolve의 값을 받는다.

• catch : error의 값을 받는다.

• finally : resolve든 reject든 무조건 실행된다.

  Promise chaining

• Promise는 chaining해서 나타낼수 있다.

 const fetchNumber = new Promise((resolve, reject) => {
  setTimeout(() => resolve(1),1000)
})

fetchNumber
  .then(num => num * 2)
  .then(num => num * 3)
  .then(num => {
    return new Promise((resolve, reject) => {
      setTimeout(()=>resolve(num-1),1000);
    })
  })
  .then(num => console.log(num))