Load data using Callback. Is there a better solution using ES6 Promises?

Question

I have a method that is called in several places in the project. I've done method. the first method call do Ajax get, cache data in class property and fire callback. Second call method only call callback with cached data. I would like to add the ability to load data synchronously. Date should be returned by the method. I added an additional parameter to call {async: false}, but I wonder if there is a better solution using ES7 promises?

This is my callback solutions.

export class loadData {
    constructor() {
        this.data = [];
    }
    getData({callback, async = true}){
        let syncData = this.data;
        if( this.data.length === 0 ){
            $.ajax({
                beforeSend: authorizationManager.addAuthorizeHeader(),
                url: apiUrl + '/Data/datadata',
                dataType: 'json',
                cache: true,
                async: async
            }).done((data)=>{
                if(async) callback(data);
                this.data = data;
                syncData = data;
            });
        } else {
            if(async) callback(this.data);
        }

        if(async === false) return syncData;
    }

}
loadDataTest = new loadData();

call async

loadDataTest.getData({
    callback: (data) =>{
        console.log(data);
    }
});

call sync

let a = loadDataTest.getData({
    async: false
});

Answer 1

Promises are almost always the better solution. Of course they are never synchronous, but that's usually the better solution as well. This is how it would look like:

export class loadData {
    constructor() {
        this.promise = null;
    }
    getData() {
        if (this.promise == null) {
            this.promise = Promise.resolve($.ajax({
                beforeSend: authorizationManager.addAuthorizeHeader(),
                url: apiUrl + '/Data/datadata',
                dataType: 'json',
                cache: true
            }));
        }
        return this.promise;
    }
}

And the call:

loadDataTest.getData().then((data) => {
    console.log(data);
});

I would like to add the ability to load data synchronously

I don't think you really want that. If all you want is synchronous-looking syntax for asynchronous functionality, have a look at async/await.