Javascript setTimeout 回调参数
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setTimeout callback argument
提问by jsnewman
Let's consider this piece of JavaScript:
让我们考虑一下这段 JavaScript：
function Person(name) {
this.name = name;
}
Person.prototype.showName = function() {
alert(this.name);
}
var mike = new Person("mike");
//mike.showName();
window.name = "window";
I don't understand the difference between the behavior of
我不明白行为之间的区别
setTimeout(mike.showName(), 5000);
and
和
setTimeout(function(){
mike.showName();
}, 5000);
Why is the behavior different? It really confuses me. Thanks.
为什么行为不同？这真的让我很困惑。谢谢。
回答by Wayne
Your question really has nothing at allto do with setTimeout. You simply need to understand the difference between a function call and a reference to a function.
你的问题真的有什么都没有做setTimeout。您只需要了解函数调用和对函数的引用之间的区别。
Consider these four assignments:
考虑这四个任务：
var one = function() { mike.showName(); };
var two = mike.showName;
var three = mike.showName();
var four = (function() { mike.showName(); })();
The first two assign a reference to a function to their respective variables. The last two, however, callfunctions (that's what the parens are for) and assign their return values to the vars on the left-hand side.
前两个将函数的引用分配给它们各自的变量。然而，最后两个调用函数（这就是括号的用途）并将它们的返回值分配给左侧的 vars。
How this relates to setTimeout:
这与 setTimeout 有何关系：
The setTimeoutfunction expects as its first argument a referenceto a function, so either oneor twoabove would be correct, but threeand fourwould not. However, it is important to note that it is not, strictly speaking, a mistaketo pass the return value of a function to setTimeout, although you'll frequently see that said.
的setTimeout函数期望作为第一个参数一个参考到的函数，因此任一one或two以上将是正确，但three并four不会。但是，重要的是要注意，严格来说，将函数的返回值传递给并不是错误setTimeout，尽管您会经常看到这样说。
This is perfectly fine, for example:
这完全没问题，例如：
function makeTimeoutFunc(param) {
return function() {
// does something with param
}
}
setTimeout(makeTimeoutFunc(), 5000);
It has nothing to do with howsetTimeoutreceives a function as its argument, but that it does.
它与如何setTimeout接收函数作为其参数无关，但它确实。
回答by JCOC611
If the accepted answer is just too long to read:
如果接受的答案太长而无法阅读：
setTimeout(mike.showName(), 5000);
This will execute whatever mike.showName()returnsafter 5,000 milliseconds.
这将执行5,000 毫秒后mike.showName()返回的任何内容。
setTimeout(function(){ mike.showName(); }, 5000);
This will execute anonymous function after 5000 milliseconds that calls mike.showName(), the actual function.
这将后5000毫秒调用执行匿名函数mike.showName()，实际功能。
Another way to achieve same effect:
实现相同效果的另一种方法：
setTimeout(mike.showName.bind(mike), 5000);
回答by ThiefMaster
It's not a performance issue. One of the ways you showed simply doesn't work (it calls the function immediately instead of when the timeout fires).
这不是性能问题。您展示的其中一种方法根本不起作用（它立即调用该函数，而不是在超时触发时调用）。
setTimeout(mike.showName(), 5000);will execute the showNamefunction and sets its return value as the timeout callback which won't work.
setTimeout(mike.showName(), 5000);将执行该showName函数并将其返回值设置为不起作用的超时回调。
setTimeout(function(){ mike.showName(); }, 5000);creates an anonymous function and sets this as the timeout callback. When the timeout fires, the function is called and calls your showName()function.
setTimeout(function(){ mike.showName(); }, 5000);创建一个匿名函数并将其设置为超时回调。当超时触发时，将调用该函数并调用您的showName()函数。
Fyi, setTimeout('mike.showName();', 5000);would also work. But don't do that- it's just as bad as using eval(). Besides that it makes your code less readable since the code in the string cannot be syntax-highlighted.
仅供参考，setTimeout('mike.showName();', 5000);也可以。但是不要那样做——这和使用eval(). 除此之外，它使您的代码可读性降低，因为字符串中的代码不能以语法高亮显示。
回答by Mike Park
setTimeout(mike.showName(), 5000);executes mike.showName()immediately and passes the return value to setTimeout()
setTimeout(mike.showName(), 5000);mike.showName()立即执行并将返回值传递给setTimeout()
setTimeout(function(){ mike.showName(); }, 5000);passes a pointer to the function instead. That way setTimeoutcan execute the function, rather than it's return value.
setTimeout(function(){ mike.showName(); }, 5000);而是传递一个指向该函数的指针。这样setTimeout就可以执行函数，而不是它的返回值。
