同步：onerror事件和n++
我刚刚阅读了一些 JavaScript 内容，并发现了一些处理加载图像过程的代码：
var n = 0;
...
for (...) {
...
image[i].onload = function { n++; };
}
这段代码安全吗？或者在访问变量 n 时会出现任何类型的竞争吗？
I'm just reading a bit into JavaScript and came over some code that handle the process of loading images:
var n = 0;
...
for (...) {
...
image[i].onload = function { n++; };
}
Is this piece of code safe or can there be any type of race while accessing the variable n?
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每个图像的 ONLOAD 事件发生的时间（相对于彼此，如果有的话）通常是不确定的。考虑浏览器缓存、并发下载或失败尝试等问题。
但是，一次只有一个 JavaScript“执行上下文”；也就是说，
n的最终值将反映调用 ONLOAD 处理程序的总次数（在给定时间）。也就是说，对于给定的页面，所有 JS 都是原子的，直到完成（在事件发生之前交给浏览器）“Web Workers”也适合此事件模型；线程和变量访问之间不存在传统的“竞争条件”。然而，任务之间的任何共享（可变）状态都可能导致竞争条件，尽管是在更高级别。
The time at which the ONLOAD event for each image is raised (relatively to each other, if at ever) is generally non-deterministic. Consider issues like browser cache or concurrent downloads, or failed attempts.
However, there is only one JavaScript "execution context" at a time; that is, the final value of
nwill reflect the total amount of times that ONLOAD handler is invoked (at that given time). That is, for a given page, all JS is atomic until it finishes (yields to the browser until an event occurs)"Web Workers" also fit within this event model; there is no traditional "race condition" between threads and variable access. However, any shared (mutable) state between tasks can result in a race-condition, albeit at at higher-levels.
这是非常安全的，因为大多数 JavaScript 都在单线程上下文中运行。唯一需要注意的是 Web Workers，但它们设计得很好，并且只能通过 JSON 消息进行通信。
This is pretty safe since most JavaScript is run in a single threaded context. The only caveat is Web Workers but they are designed quite well and can only communicate via JSON messages.