_decrementPreload and preload

p5.js Development
1

I read libraries tutorial and have a question about Example of async function for callback and preload(). I followed its code and wrote an example.

p5.prototype.loadXXX = function(callback) {
    const ret = {};
    
    setTimeout(() => {
        const data = {
            x: 1,
            y: 2
        };

        for (let prop in data) {
            ret[prop] = data[prop];
        }

        if (typeof callback == 'function') {
            callback(ret); // do the callback.
        }        

    }, 1000);

    return ret;
};
p5.prototype.registerPreloadMethod('loadXXX', p5.prototype);

When calling loadXXX in preload, the canvas only show “Loading…”. I looked into those loadXXX function in the p5.js source, they all call self._decrementPreload(). I tried to rewrite my code.

p5.prototype.loadXXX = function(callback) {
    const ret = {};
    const self = this;
    
    setTimeout(() => {
        const data = {
            x: 1,
            y: 2
        };
        console.log('Timeout');
        for (let prop in data) {
            ret[prop] = data[prop];
        }

        if (typeof callback == 'function') {
            callback(ret); // do the callback.
        }        
        self._decrementPreload();

    }, 1000);

    return ret;
};
p5.prototype.registerPreloadMethod('loadXXX', p5.prototype);

It works. I use p5.js 1.1.9. Is self._decrementPreload() an official way to do that? Does the doc require an update?