我无法理解 Array.prototype.filter() 中的“元素”参数

Question

I've been learning JavaScript through Codecademy for the past few months and right now I'm working on a credit card validator exercise.

My goal in this step is to iterate every other element in an array, and I've used the following code to do so, which works perfectly (provided by dannymac in another post):

let checkDigits = array.filter((element, index) => {
    return index % 2 === 0
    })
  console.log(checkDigits)

The thing is, even though I understand what is happening, I don't get where the " element " parameter is used. I've checked the documentation but since I'm a noob to this I still can't really see it.

I know this might look like a stupid question but I'd appreciate if somebody could briefly explain it to me.

Thank you very much.

Kind regards, Vic.

Answer 1

You are not using element in your case... The filter you have implemented will select even elements regarding their position in the array.

Here is an example where you use element:

const array = [4,12,15,9]
let checkDigits = array.filter((element, index) => {
    return element > 9
    })
  console.log(checkDigits)

/*
[12,15]
*/

Answer 2

In this situation, it's not used - it's just an unused parameter.

It's easy to construct this situation without relying on built-ins: just define a function which takes 2 arguments and only uses the second:

 const fn = (arg1, arg2) => { console.log('arg2 is', arg2); }; fn('abc', 'def');

In the case of .filter , its function signature is always the same: the first argument passed to the callback is the element being iterated over, and the second argument passed to the callback is the iteration index. Not all arguments have to be referenced in a function; a function may even define some arguments and then use none of them (though that'd be pretty weird). Here, since you can only get a reference to the 2nd argument by also assigning a name to the 1st argument, the element has to be listed in the parameter list.

A common convention for unused parameters is to give it a name of an underscore, eg:

let checkDigits = array.filter((_, index) => {

This'll work just as well as your current code, but (to some) may look less confusing.

Answer 3

if you have an array like [1, 3, 4, 5, 6, 2, 4] and you run filter on it, it will iterate over each element in the array and execute the callback function you provide. That callback function takes a few arguments, the first argument is the element with would be 1 or 3 or 4 , the second argument is the index which starts at zero and goes up the length of the array -1. In the callback function used in your code example, the element is not used because it only cares about the index of each item but since the arguments still come in the same order you have to add the element as the first argument.

here are some examples

 const arr = [1, 5, 6, 87, 2, 4, 7, 4, 6, 5] // get all the items that are divisible by 2 const filteredArr = arr.filter((element) => element % 2 == 0) console.log(filteredArr)

 const arr = [1, 5, 6, 87, 2, 4, 7, 4, 6, 5, 578, 23, 12, 1325, 78] // get all the items that are at an even index in the array and are greater than 10 const filteredArr = arr.filter((element, index) => index % 2 == 0 && element > 10) console.log(filteredArr)