Promise.All在第一次拒绝时没有碰到捕获块

Question

I'm new to promises and I'm saving multiple items to MongoDB database.

For a single item, I have a function that returns a promise, which rejects when the save to the database failed, or resolves if the save to the database succeeded:

exports.save_single_item = (itemBody, itemId) => {
return new Promise((resolve, reject) => {
    var new_item = new Item(itemBody);
    new_item.save(function (err, savedItem) {
        if (err)
            reject('ERROR');
        else {
            resolve('OK');
        }

    });
  });
};

For multiple items, I have a function that, for each item in the submitted array containing items, calls the function above. For that, I'm using this Promise.all construction:

exports.save_multiple_items = (items) => {
var actions = items.map((item) => { module.exports.save_single_item(item, item.id) });
var results = Promise.all(actions);
results.then((savedItems) => {
    console.log('ALL OK!');
}).catch((error) => {
    console.log('ERROR');

  });
};

The problem is, I'm never hitting the catch block on results.then.catch even though every promise call to save_single_item rejects. It goes straight into the then() block and prints out 'ALL OK'.

I'm getting UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 9): ERROR for every item in the array, even though I'm supposedly(?) catching it at the results.then.catch() block.

What am I missing here?

Answer 1

You are actually generating an array of undefined , because of this:

var actions = items.map((item) => { module.exports.save_single_item(item, item.id) })

If you want an array of promises, you should remove the brackets ("concise function body"):

var actions = items.map((item) => module.exports.save_single_item(item, item.id))

Or do an explicit return from the block ("block function body"):

var actions = items.map((item) => { return module.exports.save_single_item(item, item.id) })

More info here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions#Function_body