Javascript filter not working with multiple conditions

Question

Let's assume I have this array :

array = [{ 
    name: 'my post',
    categories: [{
        slug: 'a-cat',
        name: 'A Category'
    }
},
{ 
    name: 'my other post',
    categories: [{
        slug: 'another-category',
        name: 'Another Category'
    },
    {
        slug: 'a-cat',
        name: 'A Category'
    }
},
]

Now, I would like to filter it to get all element that contain the category another-category, this is what i've tried so far, but nothing ends up in the filteredArray

let filteredArray = array.filter(function (item) {
    return item.categories.forEach(function(cat) {
        return cat.slug === 'another-category'
    })
})

Any thoughts on what I've done wrong ?

const array = [{
    name: 'my post',
    categories: [{
      slug: 'a-cat',
      name: 'A Category'
    }]
  },
  {
    name: 'my other post',
    categories: [{
        slug: 'another-category',
        name: 'Another Category'
      },
      {
        slug: 'a-cat',
        name: 'A Category'
      }
    ]
  },
]


let filteredArray = array.filter(function(item) {
  return item.categories.forEach(function(cat) {
    return cat.slug === 'another-category'
  })
})

console.log(filteredArray)

forEach returns undefined which is treated as false in filter. What is your desired result? — Sebastian Simon
– Sebastian Simon, Commented Apr 28, 2018 at 18:23

Kai · Accepted Answer · 2018-04-28 18:57:09Z

Seem you misunderstand the return value of forEach. In your example code, item.category.forEach() will always return undefined when it finishes execution, that's why it went wrong.

In this case, you should use Array.some() which with return value is boolean (true/false).

let filteredArray = array.filter(function (item) {
    // this mean:
    //  if has ANY <cat>s in array item.categories has slug attribute equal 'another-category': 
    //  return current <item>
    return item.categories.some(function(cat) {
        return cat.slug === 'another-category'
    })
})

**An alternative answer is using .every():

let filteredArray = array.filter(function (item) {
        // this mean:
        //  if NOT(ALL <cat>s in array item.categories haven't .slug attribute  equal  'another-category'): 
        //  return current <item>
        return !item.categories.every(function(cat) {
            return (return cat.slug !== 'another-category')
        });
})

note: the .every() is just an additional example, in case of you guys need it in the future :)!

Vivek · Accepted Answer · 2018-04-28 18:24:55Z

1

What you need here is some. Replace forEach with some.

return item.categories.some(function(cat) {
    return cat.slug === 'another-category'
})

answered Apr 28, 2018 at 18:24

Vivek

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Comments

Prasun · Accepted Answer · 2018-04-28 18:28:38Z

1

You can use find method over categories to filter final list

let filteredArray = array.filter(item => {
    return item.categories.find( category => category.slug === 'another-category');
});

answered Apr 28, 2018 at 18:28

Prasun

5,0132 gold badges23 silver badges23 bronze badges

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