Javascript Promise Ready

Question

I have a class (LoadClass) which inits when the page loads, and opens an SQLite database in my case and executes a couple of queries. Now I want other components to only execute code when LoadClass has previously finished loading the database, but this must happen anytime. For obvious reasons, I only want to open my databases once. It should work something like this:

class LoadClass {

    // This executes directly on page load
    constructor() {
        this.load_my_database().then(function() {
            // My database finished loading, now my buttons may work.
        })
    }
}

function user_presses_button() {
    LoadClass.ready().then(function() {
        // Only now something
    })
}

How would I go about doing this using Promises?

So all you are asking for is how to write a singleton? I always thought Angular services provided that functionality out of the box. — Bergi, Feb 16 '17 at 17:20
You really [shouldn't make an asynchronous call from within a constructor](http://stackoverflow.com/a/24686979/1048572). — Bergi, Feb 16 '17 at 17:21
store the database loading promise and re-use it as needed. Can call `then()` on same promise as often as you want — charlietfl, Feb 16 '17 at 17:26
Please read [ask]. Key phrases: "Search, and research" and "Explain ... any difficulties that have prevented you from solving it yourself". — Heretic Monkey, Feb 16 '17 at 17:26

score 1 · Accepted Answer · answered Feb 16 '17 at 17:42

class LoadClass {

    private _ready_promise; 

    constructor() {}

    // This executes directly on page load
    page_load() {
        this._ready_promise = this.load_my_database();
    }

    load_my_database() {
        return new Promise(function(resolve) {
            // Do something...
            // Now my database finished loading, everything else may work.
            return resolve();
        })
    }

    ready() {
        return this._ready_promise;
    }
}

function user_presses_button() {
    LoadClass.ready().then(function() {
        // Only now something
    })
}

Javascript Promise Ready

1 Answers1