javascript - 是什么使这两个函数分别为 'public' 和 'private'，这意味着什么？

Question

这两个函数来 self 正在学习的类(class) (https://www.udacity.com/course/front-end-web-developer-nanodegree--nd001)。代码和注释来自类(class)提供者:

/* This is the publicly accessible image loading function. It accepts
 * an array of strings pointing to image files or a string for a single
 * image. It will then call our private image loading function accordingly.
 */
function load(urlOrArr) {
    if(urlOrArr instanceof Array) {
        /* If the developer passed in an array of images
         * loop through each value and call our image
         * loader on that image file
         */
        urlOrArr.forEach(function(url) {
            _load(url);
        });
    } else {
        /* The developer did not pass an array to this function,
         * assume the value is a string and call our image loader
         * directly.
         */
        _load(urlOrArr);
    }
}

/* This is our private image loader function, it is
 * called by the public image loader function.
 */
function _load(url) {
    if(resourceCache[url]) {
        /* If this URL has been previously loaded it will exist within
         * our resourceCache array. Just return that image rather
         * re-loading the image.
         */
        return resourceCache[url];
    } else {
        /* This URL has not been previously loaded and is not present
         * within our cache; we'll need to load this image.
         */
        var img = new Image();
        img.onload = function() {
            /* Once our image has properly loaded, add it to our cache
             * so that we can simply return this image if the developer
             * attempts to load this file in the future.
             */
            resourceCache[url] = img;

            /* Once the image is actually loaded and properly cached,
             * call all of the onReady() callbacks we have defined.
             */
            if(isReady()) {
                readyCallbacks.forEach(function(func) { func(); });
            }
        };

        /* Set the initial cache value to false, this will change when
         * the image's onload event handler is called. Finally, point
         * the image's src attribute to the passed in URL.
         */
        resourceCache[url] = false;
        img.src = url;
    }
}

为什么 load() 是“公开访问的”，而 _load() 是“私有(private)的”？在这种情况下，公共(public)/私有(private)意味着什么？

如果您需要，完整文件位于https://github.com/YolkFolkDizzy/frontend-nanodegree-arcade-game/blob/master/js/resources.js。

Answer 1

它是私有(private)的，因为它不能被直接调用...见第 105 行:

window.Resources = {
    load: load,
    get: get,
    onReady: onReady,
    isReady: isReady
};

由于该方法是在范围内声明的，因此在其他任何地方都将不可用。

可以看到里面写了代码:

(function() {
...
})()

它强制将任何函数声明或变量声明附加到当前作用域。如果没有这个，变量将附加到最近的当前对象，通常是窗口。因此 _load 永远不会导出，调用它的唯一方法是调用 Resource 对象中的 window 导出的方法之一。

• 公共(public)是指可以从外部调用某些东西。
• 私有(private)是指某些东西只能从内部调用。

在 Javascript 中，私有(private)属性通常隐藏在一个作用域中，该作用域仅对在该作用域内创建的函数可用。