Why flatten isn't working?

Question

I have read the code in how to flatten an array in MDN in JS. And works fine, but I don't understand why isn't working in this case:

const data = [null, ['a', 'b', 'c']]
const flattened = data.reduce((acc, cur) => {
    if(null != cur)
        acc.concat(cur)
}, [])

And this error:

TypeError: Cannot read property 'concat' of undefined

How to correct this?

If the purpose is to filter null values (or false values maybe?) out, you could also do that early one by using Array#sort: `data.filter(cur => cur !== null).reduce((acc, cur) => acc.concat(cur), []);` — thibmaek, May 14 '17 at 00:24
@naomik While the subject matter is addressed at linked Question, the specific issue at Question is not addressed by an Answer at linked Question. Is there a Question/Answer addressing no value being returned from `Array.prototype.reduce()` callback function? — guest271314, May 14 '17 at 00:38
@thibmaek I wanted to iterate only once over the array, that's why I choose just the reduce. If I had used a filter before it would iterate over again by the reduce method. — Lucas Almeida Carotta, May 14 '17 at 19:06

score 4 · Answer 1 · answered May 14 '17 at 00:18

4

No value is returned from function passed to .reduce()

const data = [null, ['a', 'b', 'c']]
const flattened = data.reduce((acc, cur) => {
        if (cur !== null)
          acc = acc.concat(cur);
        return acc   
}, []);

console.log(flattened);

answered May 14 '17 at 00:18

guest271314

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@LucasAlmeidaCarotta Also `acc.concat(cur)` should be `acc = acc.concat(cur)` or `acc.push(...cur)` – guest271314 May 14 '17 at 00:36

Why flatten isn't working?

1 Answers1