在javascript中传递函数和函数调用本身之间的区别是什么？

Question

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);

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

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

Why?

Answer 1

在第一个例子中， setTimeout的第一个参数被赋予$.get （错误）的结果 ，而在第二个例子中它实际上是接收一个类型为function的参数，它将被正确地评估为每个x的一组javascript语句毫秒。

Answer 2

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.

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);

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

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);

Answer 3

You shouldn't be passing the result of a function call to setTimeout - there's no sense in doing that. First argument should be the function itself, not the call.

Why it fires continuously - a strange side-effect, who knows :)