How is this working? I am having a hard time understand this.
const arr1 = [{id:1,name:jhon},{id:2,name:max},{id:3,name:fer}];
const arr2 = [8, 9];
var diffArray = arr2.filter(x => !arr1.filter(y => y.id === x).length);
console.log(diffArray);From MDN The filter() method creates a new array with all elements that pass the test implemented by the provided function.
Code x => some_code is called an arrow function and can be translated to: function(x) {return some_code}
Basically what is done here is that arr2.filter() will return all elements of said arr2 that pass the condition. Condition here is, that the length of filtered arr1 must be zero (meaning, that the match was not found). Length of array as numerical value can be used as true/false and can be negated with !. That's whats going on here:
arr1.filter(y => y.id === x) // means, give me elements of arr1, that are the same as in array 2
arr1.filter(y => y.id === x).length // means, the length of said array of elements
!arr1.filter(y => y.id === x).length // means, if length == 0 make it true and if more than 0 -> make it false
