英文:
Why are event argument optional in anonymous functions in Javascript?
问题
I understand. Here's the translated code:
我只是试图理解这段代码背后的逻辑：
window.onkeydown = function () {
handler(event);
};
function handler(event)
{
console.log(event.key); // 这个可以工作！
}
事件处理程序不应该在匿名函数中包含事件参数吗？像这样：
window.onkeydown = function (event) {
handler(event);
};
我明白浏览器会自动将事件传递给处理程序，但我仍然觉得在没有参数的匿名函数中分配它仍然能够工作很奇怪。这是正常行为吗？
英文:
I'm just trying to understand the logic behind this code:
window.onkeydown = function () {
handler(event);
};
function handler(event)
{
console.log(event.key); // this works!
}
Shouldn't the event handler be declared with the event argument included in the anonymous function? Like so:
window.onkeydown = function (event) {
handler(event);
};
I understand that browsers automatically pass the event to the handler, but still I find it weird that assigning an anonymous function without arguments still works. Is this a normal behaviour?
答案1
得分: 1
为什么在JavaScript的匿名函数中事件参数是可选的？
它们并不是在所有浏览器中都可选的。Microsoft 曾经有一个 全局 event，在调用处理程序之前，使用他们的 attachEvent（和 onxyz）处理程序将其设置为当前事件。DOM 标准采用了另一种方式：将 event 作为参数传递。
Chrome 和一些其他浏览器都支持两种方式，以便 Microsoft 特定的代码在其中正常工作。但并非所有浏览器都支持。例如，Firefox 不支持，除非在用户首选项中设置特殊偏好项（详情）。
关键要点是：使用现代事件处理方式（例如 addEventListener 等），它在除了 IE8 及更早版本之外的所有浏览器中都受支持，并声明函数的 event 参数：
window.addEventListener("keydown", handler);
// ...
function handler(event) {
// ...
}
或者（函数名称是可选的，但在错误调用堆栈中很有用）：
window.addEventListener("keydown", function handler(event) {
// ...
});
英文:
> Why are event argument optional in anonymous functions in Javascript?
They aren't, cross-browser. Microsoft had a global event that was set to the current event before calling handlers using their attachEvent (and onxyz) handlers. The DOM standard went another way: passing event as an argument.
Chrome and some other browsers do both, so that Microsoft-specific code works on them. But not all browsers do. Firefox doesn't, for instance, unless you set a special preference in your user preferences (details).
The key takeaway is: Use modern event handling (addEventListener, etc.), which is supported in all browsers other than IE8 and earlier, and declare the event parameter to the function:
window.addEventListener("keydown", handler);
// ...
function handler(event) {
// ...
}
or (the function name is optional, but useful in error call stacks):
window.addEventListener("keydown", function handler(event) {
// ...
});
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