作为参数传递的函数是否总是回调？ JavaScript的

Question

If I have the code below, where I pass two functions as a parameters into the function sayHi , is this an example of a callback?

I notice there are two ways of running these 'parameter functions': either as below, we I call the functions where they are defined (as arguments), or alternatively where I call the parameter in the sayHi function. Would this be the difference between a callback and an anonymous function?

function sayHi(name, testForTrue) {
    if (testForTrue == true) {
        console.log(name);
    }
}

sayHi(function() {
    return 'Zach'
}(), function() {
    return true;
}());

Another way I could get the same result, is as below. In this case I am evaluating the functions at a different time? Is there any practical difference between the two?

function sayHi(name, testForTrue) {
    if (testForTrue() == true) {
        console.log(name());
    }
}

sayHi(function() {
    return 'Zach'
}, function() {
    return true;
});

Answer 1

Yes, functions passed as parameters are always callbacks, even if the intention is that the function is called synchronously (cf Array.prototype.map ) rather than asynchronously (cf window.setTimeout ).

In your first code block you aren't of course actually passing functions. You have two immediately invoked function expressions , where the key part in this context is immediately invoked . The function expressions are called at the point they appear in the code and only the results of those expressions are passed to sayHi .

Answer 2

In your first example you're not passing functions, but values; in other words

(function(){ return 3; })()

is just the integer 3.

It is a value obtained calling immediately a function, but this is irrelevant.

When you pass a callback it's the receiver that will call it (or pass it to some other function) and the code will be executed later, and not at the call site.

Answer 3

I guess that would depend on what your callback function is actually doing.

In your examples, all you're really doing is returning a value. That's not really a "function", it's returning one fixed value every single time.

If your function is actually doing a process, or returning a varied result, then I would personally consider it a callback. (The name of it is self-explanatory, really). Your script shouldn't rely on it, rather have it be a handler for the result of the function.

For instance, something like this would be what I consider a callback function:

function doSomething(callback) {
  var userInput = prompt("hello, enter a number 1-10"),
      hasWon = false;

  if (userInput === "3") hasWon = true;

  callback(hasWon);
};

With this provided, we can call it like this:

doSomething(function(hasWon){
  if (hasWon) alert("Congratz! You guessed my lucky number!")
});