setTimeout回调参数 [英] setTimeout callback argument

问题描述

让我们考虑这段JavaScript：

Let's consider this piece of JavaScript:

function Person(name) {
    this.name = name;
}

Person.prototype.showName = function() {
    alert(this.name);
}


var mike = new Person("mike");
//mike.showName();  

window.name = "window"; 

我不明白行为的区别

setTimeout(mike.showName(), 5000);

和

setTimeout(function(){
    mike.showName();
}, 5000);

为什么行为会有所不同？这真让我困惑。谢谢。

Why is the behavior different? It really confuses me. Thanks.

推荐答案

您的问题确实什么都没有与有关的setTimeout 。您只需要了解函数调用和函数引用之间的区别。

Your question really has nothing at all to do with setTimeout. You simply need to understand the difference between a function call and a reference to a function.

考虑这四个分配：

var one = function() { mike.showName(); };
var two = mike.showName;
var three = mike.showName();
var four = (function() { mike.showName(); })();

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parens的用途）并将它们的返回值分配给左侧的变量。

The first two assign a reference to a function to their respective variables. The last two, however, call functions (that's what the parens are for) and assign their return values to the vars on the left-hand side.

这与setTimeout的关系如何：

setTimeout 函数需要作为其第一个参数引用到一个函数，所以上面的一个或两个是正确的，但三和四不会。但是，重要的是要注意，严格来说，错误并不是将函数的返回值传递给 setTimeout ，尽管你经常会看到这样说。

The setTimeout function expects as its first argument a reference to a function, so either one or two above would be correct, but three and four would not. However, it is important to note that it is not, strictly speaking, a mistake to pass the return value of a function to setTimeout, although you'll frequently see that said.

这很好，例如：

function makeTimeoutFunc(param) {
    return function() {
        // does something with param
    }
}

setTimeout(makeTimeoutFunc(), 5000);

它与如何 setTimeout无关接收一个函数作为其参数，但它接收。

It has nothing to do with how setTimeout receives a function as its argument, but that it does.

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