javascript - "this._events || (this._events = {});"是什么意思？

Question

我已经开始学习 Backbone.js。目前我的 JavaScript 技能不太好。我开始检查 backbone.js 文件，发现一行奇怪的代码，我无法弄清楚其用途。代码示例(如果您需要更多上下文，请为开发人员手动下载 backbone.js 并查看第 80 行):

var Events = Backbone.Events = {

  // Bind an event to a `callback` function. Passing `"all"` will bind
  // the callback to all events fired.
  on: function(name, callback, context) {
    if (!eventsApi(this, 'on', name, [callback, context]) || !callback) return this;
    this._events || (this._events = {});
    var events = this._events[name] || (this._events[name] = []);
    events.push({callback: callback, context: context, ctx: context || this});
    return this;
  },

行 this._events || 是什么意思(this._events = {}); 是什么意思？对我来说，_events 看起来像一个内部变量，但是 (this._events = {}) 用于赋值还是或比较？还是 || 在这种情况下是完全不同的操作？

Answer 1

这是一个使用 javascripts“虚假”评估的技巧。它与以下内容相同:

if (this._events) {
    // do nothing, this._events is already defined
} else {
    this._events = {};
}

var events = this._events[name] || 这行也是如此(this._events[name] = []); 可以翻译成

var events;
if (this._events[name]) {
    events = this._events[name];
} else {
    this._events[name] = [];
    events = this._events[name];
}