从Javascript“过滤器”函数返回“空”值

Question

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?

Answer 1

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])); 

Answer 2

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.