在javascript中传递函数和函数调用本身之间的区别是什么？ [英] What's the differenct between passing a function and the function call itself in javascript?

问题描述

在我正在构建的应用程序中，我正在轮询状态更新，我注意到如果调用如下，则超时会持续触发：

In the application I'm building I'm polling for a status update and I have noticed that if the call is made as follows the timeout fires continuously:

setTimeout($.get("http://localhost:8080/status", function(data) { UpdateStatus(data);}), 1000);

如果使用函数而不是每1000毫秒触发超时：

While if use a function instead the timeout fires every 1000 ms:

 setTimeout(function() {$.get("http://localhost:8080/status", function(data) { UpdateStatus(data);})}, 1000);

为什么？

推荐答案

在第一个示例中，您调用 $。获取然后将其返回值传递到的setTimeout 。在第二个例子中，你根本没有调用函数;你给 setTimeout 一个它稍后会调用的函数，然后调用 $。get 为你。

In the first example, you're calling $.get and then passing its return value into setTimeout. In the second example, you're not calling the function at all; you're giving setTimeout a function that it will call later, which will then call $.get for you.

使用更简单的测试用例更容易看到：

This is easier to see with a simpler test case:

function test() {
    alert("Hi there!");
}

// WRONG, *calls* `test` immediately, passes its return value to `setTimeout`:
setTimeout(test(), 1000);

// Right, passes a reference to `test` to `setTimeout`
setTimeout(test, 1000);

请注意，第一个有圆括号（（）），第二个没有。

Note that the first one has parentheses (()), the second one doesn't.

当你想将参数传递给函数时，你必须通过定义另一个函数间接地做到这一点：

When you want to pass parameters to the function, you have to do it indirectly by defining another function:

function test(msg) {
    alert(msg);
}

// WRONG, *calls* `test` immediately, passes its return value to `setTimeout`:
setTimeout(test("Hi there!"), 1000);

// Right, passes a reference to a function (that will call `test` when called) to `setTimeout`
setTimeout(function() { test("Hi there!"); }, 1000);

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