Array.filter

Question

Can someone please explain this?

var c = new Array(1, 2, 3, 4);

var b = c.filter(function(a) {
return (a % 2 - 1);
})

alert(b[0]); // 2

Why is the result 2?

Answer 1

filter() method does not execute the function for empty elements.
1%2-1 = 0(an empty value)
2%2-1 = -1(not an empty)

So, it returns the even numbers from array c hence, 2 is the first element.

Answer 2

filter() method iterates an array, invokes an aggregate function on each iteration, passing one array element to the aggregate function at a time (sequentially).

An element passes the qualification when the aggregate function, being invoked with the element as argument (here represented by <a>), returns truthy value.

+ An element that passes the qualification will be included in filtered array.

<a> = 1 (1st element)
a % 2 - 1 evaluates as ((1 % 2) - 1), resulted as 0
(1) doesn't pass the test (zero is falsy)

<a> = 2 (2nd element)
a % 2 - 1 evaluates as ((2 % 2) - 1), resulted as -1
(2) passes the test (-1 is truthy)

<a> = 3 (3rd element)
a % 2 - 1 evaluates as ((3 % 2) - 1), resulted as 0
(3) doesn't pass the test (zero is falsy)

<a> = 4 (4th element)
a % 2 - 1 evaluates as ((4 % 2) - 1), resulted as -1
(4) passes the test (-1 is truthy)

The array returned by filter() method is [ 2, 4 ]. Being subscripted with index 0, the 1st element (2) will be displayed in the alert() box.

Answer 3

David,
Very welcome
My pleasure 👌

Answer 4

Ipang very well explained, thanks a lot for your time :)

Answer 5

Simba thanks.