Let's say i have the following array: arr = [1, 2, 0, null, 4], and i want to filter all the "0" in the array. An example code would be:

arr.filter((val) => {return val !== 0})
//>> [1, 2, null, 4]

It works just fine. But if i use an if statement instead, the "null" value isn't returned:

arr.filter((val) => {if(val !== 0) return val})
//>> [1, 2, 4]

It only works if i return an array containing the "value":

arr.filter((val) => {if(val !== 0) return [val]})
//>> [1, 2, null, 4]

Could anybody explain why when using an if statement the "null" only is returned if it is in an array?

Let's say i have the following array: arr = [1, 2, 0, null, 4], and i want to filter all the "0" in the array. An example code would be:

arr.filter((val) => {return val !== 0})
//>> [1, 2, null, 4]

It works just fine. But if i use an if statement instead, the "null" value isn't returned:

arr.filter((val) => {if(val !== 0) return val})
//>> [1, 2, 4]

It only works if i return an array containing the "value":

arr.filter((val) => {if(val !== 0) return [val]})
//>> [1, 2, null, 4]

Could anybody explain why when using an if statement the "null" only is returned if it is in an array?

• javascript
• arrays

• well filter is supposed to return true or false.... – epascarello Commented Feb 8, 2019 at 23:53

2 Answers 2

An item will be included in the resulting array if the value returned from the filter callback is truthy.

return val !== 0

will return false if val is 0, and true otherwise.

if(val !== 0) return val

will return undefined if the val is 0, and will return the val otherwise. So, if the val is not 0, but is still falsey (like null), it won't be included in the result.

But arrays are always truthy (no matter what values they contain), so

if(val !== 0) return [val]}

will always result in non-0 values being included in the final array.

console.log(Boolean(null));
console.log(Boolean([null]));

Well filter callback is supposed to return true or false.

You are not returning true or false. So it converts your item into a boolean.

So null is falsly an array would be truthy.