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Javascript Promise

scdan0624 profile image SCDan0624 ・1 min read

How to create a Javascript promise

A promise is a function that will complete a task, usually asynchronously, in the future. A promise is always in one of three states:

pending: initial state, neither resolved or rejected

fulfilled: operation was completed successfully

rejected: operation has failed
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to create a promise you use the following syntax:

const myPromise = new Promise((resolve, reject) => {

})

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How to complete a promise

To actually complete a promise you must add conditions in the function. Using the previous example this is how you would complete the promise:

const myPromise = new Promise((resolve, reject) => {
  if(condition){
  resolve("Promise has been fulfilled")
  } else {
  reject("Promise was rejected")
  }
})

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Handle a fulfilled promise with then

Often promises are used with server requests since they can take an unknown amount of time. After you fulfill a promise you usually want to do something with the response from the server. To do this you can use the then method. The then method is executed immediately are the promise is resolved. Here is an example:

myPromise.then(result => {
// Write what you will do with result here
console.log(result)
})
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Handle a rejected promise with catch

Catch is used when your promise has been rejected. It will be executed immediately after a the reject method is called.

Here is the syntax:

myPromise.catch(error => {
// Write what you will do with error here
console.log(error)
})
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