Filter not behaving as expected with 0 in an array. Javascript

Question

I have an array of random integers like this: [ 1, 2, 3, 4, 0 ], I need to return a new array of either the odd or the even numbers depending on a wanted condition.

I tried the following filter method:

const result = array.filter(item => {
    const test = item % 2;
    if(wanted === 'odd' && test > 0) return item;
    if(wanted === 'even' && test === 0) return item;
  })

and had expected this to work but it only ever returns [2, 4] and never [0, 2, 4] when the wanted is even.

Any thoughts on why this might be the case would be greatly appreciated.

test will always be either 1 or 0. Testing > 0 is confusing when you're really just looking for 1. — Barmar
– Barmar, Commented Sep 28, 2018 at 23:20

Mark · Accepted Answer · 2018-09-28 23:22:25Z

2

You need to return a boolean from your filter callback, not the item. When you return the item it mostly works, but if the item is zero, that's interpreted as false

let arr = [ 1, 2, 3, 4, 0 ]
let wanted = 'even'
const result = arr.filter(item => {
    const test = item % 2;
    if(wanted === 'odd' && test > 0) return true;
    if(wanted === 'even' && test === 0) return true;
  })
console.log(result)

You can simplify any just returning the booleans created by your tests:

let arr = [ 1, 2, 3, 4, 0 ]
let wanted = 'even'
const result = arr.filter(item => {
    if (wanted === 'odd') return item % 2
    if (wanted === 'even') return !(item % 2) 
  })
console.log(result)

edited Sep 28, 2018 at 23:22

answered Sep 28, 2018 at 23:15

Mark

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3 Comments

ibrahim mahrir Over a year ago

You can simplify it even more using XOR (binary XOR in fact, which works because the values used are 0s and 1s): arr.filter(item => wanted === "odd" ^ item % 2)

Mark Over a year ago

I would have never thought of that @ibrahimmahrir. That's pretty cool.

Rennzie Over a year ago

Thanks guys, that is really helpful and has upgrade my understanding of filter.

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