Promises in javascript(🔖)

#beginners #javascript #webdev #tutorial

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.