JS [promise]

'use strict';

// Promise is a JavaScript Object for asynchronous operation.

// State: pending -> fulfilled or rejected

// Producer vs Consumer

// 1. Producer

// when new Promise is created, the executor runs automatically.

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

    //doing some heavy work (network, read files)

    console.log('doing something...');

    setTimeout(() => {

        resolve('ellie');

        //reject(New Error('no network'));

    }, 2000);

});

// 2. Consummers: then, catch, finally

promise.

then((value)=>{

    console.log(value);

})

.catch(error=>{

    console.log(error);

})

.finally(()=>console.log('finally'));

// 3. Promise channing

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));

})

// 4. Error Handing

const getHen = ()=>

new Promise((resolve,reject)=>{

    setTimeout(() => {

        resolve('chicken');

    }, 1000);

});

const getEgg = hen=>

new Promise((resolve,reject)=>{

    setTimeout(() => {

        resolve(`${hen} => egg`)

    }, 1000);

});

const cook = egg =>

    new Promise((resolve,reject) =>{

        setTimeout(() => {

            resolve(`${egg} => fried egg`)

        }, 1000);

    })

// getHen()

// .then(hen=>getEgg(hen))

// .then(egg=>cook(egg))

// .then(meal=>console.log(meal));

getHen()

.then(getEgg)

.catch(error=>{

    return 'bread';

})

.then(cook)

.then(console.log);