Upon attaching a listener to the Firebase Database (using on() in this scenario), the process of loading data from the database is initiated. As this operation may require some time, the JavaScript code will proceed with execution, leading to the printing of an empty array initially. Subsequently, when the data becomes accessible from the server, the callback function is triggered and incorporates the data into the array.
For better comprehension, consider incorporating log statements:
createprofile() {
console.log("Initiating listener");
this._UserRef.on("value", function(snapshot) {
console.log("Data received");
let items = [];
snapshot.forEach(function(childSnapshot) {
let item = childSnapshot.val();
item['key'] = childSnapshot.key;
items.push(item);
});
this.kitems = items;
}.bind(this));
console.log("Post listener setup");
}
The sequence of output will be as follows:
Initiating listener
Post listener setup
Data received
This behavior might differ from your expectations but aligns with the characteristics of modern internet-based programming. Such occurrences are common with most APIs.
An effective approach involves restructuring your code when handling asynchronous APIs. In traditional programming, the sequence typically revolves around "obtain A first, then execute B". However, for asynchronous APIs, it's more about "upon receiving A, perform B with it". In this context, you should relocate the code that depends on kitems within the callback function:
createprofile() {
this._UserRef.on("value", function(snapshot) {
let items = [];
snapshot.forEach(function(childSnapshot) {
let item = childSnapshot.val();
item['key'] = childSnapshot.key;
items.push(item);
});
this.kitems = items;
console.log(this.kitems);
}.bind(this));
}
Now, the logging of kitems will only occur once the data retrieval from the server is complete. Moreover, Firebase Database ensures data synchronization, prompting the callback to run whenever the data undergoes changes.
To enhance reusability, it's common practice to introduce a callback function within the data loading process:
createProfileAndThen(callback) {
this._UserRef.on("value", function(snapshot) {
let items = [];
snapshot.forEach(function(childSnapshot) {
let item = childSnapshot.val();
item['key'] = childSnapshot.key;
items.push(item);
});
this.kitems = items;
callback(this.kitems);
}.bind(this));
}
createProfileAndThen(function(kitems) {
console.log(kitems);
});
This strategy closely resembles the callback mechanism utilized in Firebase's on() function but can be customized according to specific requirements.