为什么在成功函数的末尾不需要'（）'？

Question

I got the following code:

navigator.geolocation.getCurrentPosition(
  successFunction
);

I was wondering why it is not required to use the '()' at the end of the succesFunction? Isn't it required to call a function with the '()' --> somefunction() What's the logic behind it?

Answer 1

因为您传递的是函数引用，而不是函数的返回值。

Answer 2

A function is a thing that can be invoked—where it runs code and returns a result. You invoke a function by using the parentheses. But, if you want to talk about the thing, if you want to pass the function around so that someone else can invoke it, you leave off the parentheses.

In this case, the getCurrentPosition() function is asking you to give it a function so that it can call it later on. So instead of invoking successFunction, you pass along a reference to it.

If you instead wrote:

navigator.geolocation.getCurrentPosition(
  successFunction()
);

…then what would happen is successFunction() would be invoked—the code of the function would be run, and possibly return a value—and THEN you would invoke getCurrentPosition() , passing along as an argument whatever value successFunction returned.

Answer 3

In JavaScript you can pass functions as objects:

Try executing this example:

function myFunction() {
    console.log('hello');
}

function anotherFunction(functionParameter) {
    console.log('executing function passed as argument:');
    functionParameter();
}

anotherFunction(myFunction);

This code will print:

"executing function passed as argument:"
"hello"