Promise is an object that bundles a provider and consumer. To understand this, remember how this keyword was encountered in async javascript blog?
Promises are used to keep track of whether a successful output was received from a piece of code or not.
Promises has two callbacks, either one of which is executed depending on the output received- resolve(value) & reject(error).
Resolve is for successful run while reject is for errors received.
const MyPromise=new Promise((resolve, reject)=>{
const rand=Math.floor(Math.random()*2);
console.log(rand)
if(rand===0)
resolve();
else
reject()
})
MyPromise
.then(()=>{console.log("success")})
.catch(()=>{console.log("Error")})
Here we are working with a random number case, where it either returns 1 or 2. .then
handles succesful responses and .catch
works with errors. These are used to access internal Promise properties called state
and result
.
state indicates status of the job that needs to be done. Pending, fulfilled or rejected are the three indicators.
result is initially empty but gets a value when either resolved or an error when rejected.
Only the first call to resolve/reject is taken and once the value for state and result is set it's final. More on promises by javascript.info
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Top comments (2)
your Promise params position is reverse
Corrected, thanks!