如何在此函数中传递参数？

Question

Doing a text book exercise where:

let arr = [1, 2, 3, 4, 5, 6, 7];

function inBetween(a, b) {
  return function(x) {
    return x >= a && x <= b;
  };
}

alert( arr.filter(inBetween(3, 6)) ); // 3,4,5,6

The textbook also states that the filter syntax to be:

let results = arr.filter(function(item, index, array) {
  // should return true if the item passes the filter
});

So I'm not completely understanding how the inBetween(a,b) function works... like in this line:

arr.filter(inBetween(3,6))

It seems to me like a is the item parameter, b is in the index parameter, but obviously that's not how it's working... Can someone break down this syntax and why it's working?

Answer 1

So the filter method accepts a function that should return true or false, whether to keep the item or not.

In this example, instead of writing that function inside the filter, it's written outside and passed in. However you can still think about it like this:

let results = arr.filter(function(item, index, array) {
    return item >= 3 && item <= 6;
});

The reason you would define inBetween outside the filter is so you can pass in values instead of hard coding them into the filter like above.

When you call inBetween(3,6) returned is :

function(x) {
    return x >= 3 && x <= 6;
}

Like above that's then put into the filter (just without the index/array parameter since they are not needed:

let results = arr.filter(function(x) {
    return x >= 3 && x <= 6;
});

Answer 2

a和b ， 3 ， 6在的范围限定inBetween和引用的返回的匿名函数，它是回调内.filter()由所指示的4castle ， x是item在回调函数