如何将promise结果传递给外部函数？

Question

function readData(field) {
    var profileDatabase = firebase.database();
    var user = firebase.auth().currentUser;
    var name = user.email.substring(0, user.email.indexOf("@"));
    return profileDatabase.ref('/users/' + name + 'id').once('value').then(function (snapshot) {
        return snapshot.val()[field];
    });
}

I want the readData function to return snapshot.val()[field], but Promise is asynchronous. How can I resolve that?

Answer 1

What you've done is exactly right: Return it from the then callback, and return the result of calling then from readData . The promise then creates will resolve itself with that value, and the caller using readData will see it in their then callback.

readData(field).then(function(value) {
    // Here, `value` is the value from `snapshot.val()[field]`
});

Here's an example (using setTimeout to simulate the database operation):

 function fakeDatabaseOperation() { return new Promise(function(resolve) { setTimeout(function() { resolve({ val: function() { return {foo: "bar"}; } }); }, 500); }); } function readData(field) { /* var profileDatabase = firebase.database(); var user = firebase.auth().currentUser; var name = user.email.substring(0, user.email.indexOf("@")); */ return fakeDatabaseOperation().then(function (snapshot) { console.log("Got snapshot here"); return snapshot.val()[field]; }); } readData("foo").then(function(val) { console.log("Value here: " + val); });