兑现诺言，兑现诺言

Question

function task1(fullfill, reject) {
console.log('Task1 start');
setTimeout(function() {
    console.log('Task1 end');
    //fullfill('Task1 result');
    reject('Error msg');
}, 300);
}
function fullfilled(result) {
     console.log('fullfilled : ', result);
}
function rejected(err) {
     console.log('rejected : ', err);
}
new Promise(task1).then(fullfilled, rejected);

I just started node.js and was studying about promise module(?). It could be a very basic question but I couldn't found out where the fulfilled and rejected method gets the parameter's value.

Answer 1

The then() method returns a Promise. It takes up to two arguments: callback functions for the success and failure cases of the Promise.

p.then(onFulfilled[, onRejected]);

p.then(function(value) {
  // fulfillment
}, function(reason) {
  // rejection
});

onFulfilled A Function called if the Promise is fulfilled. This function has one argument, the fulfillment value. onRejected Optional A Function called if the Promise is rejected. This function has one argument, the rejection reason.

let p = function(){
    return new Promise(function(resolve, reject) {
        if(condition){
            // this arg would be caught in onFulfilled
            resolve(arg);
        }
        else{
            // this arg would be caught in onRejected
            reject(arg2);
        }
    })
}

take a look at p for clarity

Answer 2

To add to @marvel308's answer, whatever you call resolve() with is available in the then clause and whatever you call the reject() with is available in the catch clause.

Consider:

function makePromise(condition) {
  return new Promise(function(resolve, reject) {
    condition 
    ? resolve('2 + 2 = 4') 
    : reject('No, wait. Maybe it is 5.')
  })
}

Now, if we call this function with the resolve case:

makePromise(true)
.then(x => console.log(x)) // 2 + 2 = 4
.catch(x => console.log(x))

And with the reject case:

makePromise(false)
.then(x => console.log(x)) 
.catch(x => console.log(x)) // No, wait. Maybe it is 5.