回调函数的参数如何工作？

Question

So I'm trying to understand functional programming and higher order functions in specific. What I never clearly understood, was how callback function's parameters have a connection to, for example, an array?

To be more clear, let's take this code example:

var friends = ["Mike", "Stacy", "Andy", "Rick"];

friends.forEach(function (eachName, index){
console.log(index + 1 + ". " + eachName); // 1. Mike, 2. Stacy, 3. Andy, 4. Rick
});

How a callback parameter eachName knows that it means to return the item from array and index is supposed to return an index of array? How the connection between an array and callback parameter works?

Answer 1

the forEach method is calling or invoking your callback function. To be more specific let's see how the forEach work.

function forEach(arr, func) {
  for (var i = 0; i < arr.length; i++) {
    func(arr[i], i);
  }
}

Now you can do something like this.

 function forEach(arr, func) { for (var i = 0; i < arr.length; i++) { func(arr[i], i); } } var cars = ["BMW", "Tata Nano", "Lamborghini"]; forEach(cars, function (car, index) { alert(car + " " + index); }); 

Answer 2

The API does it for you. In this case, array.forEach 's callback is always set to accept 3 arguments: the current item, the current item's index in the array, and the array itself.

One example of an API that has a different argument signature is array.reduce . It is set to accept 4 arguments: The accumulator, the current item, the current item's index and the array itself.