How Reduce function really work in JavaScript?

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I'm trying to use reduce function in order to find the max length array in arrays so I tried this approach at first but it didn't work

const arrays = [[1],[3,3,4],[3,4],[4,5],[6]]
let maxLength = arrays.reduce( (acc, cur) => {
    return acc.length > cur.length ? acc.length : cur.length
})

console.log(maxLength) // output : 1

so I tried to get the length in another way and it worked

const arrays = [[1],[3,3,4],[3,4],[4,5],[6]]
let maxLength = arrays.reduce( (acc, cur) => {
    return acc.length > cur.length ? acc : cur
}).length

console.log(maxLength) // output : 3

Can someone Explain what is the difference between the two approaches ?

maxlength reduce arrays javascript
2 Answers

If you're interested in the length of the longest array, store the length in the accumulator (as in the first example); if you're interested in the array with the longest length, store the array in the accumulator (as in the second example). What you can't do is mix and match.

Get the length of the longest array

In this case, you need to provide an initial value (e.g. 0) to reduce().

const arrays = [[1],[3,3,4],[3,4],[4,5],[6]];

const max = arrays.reduce((a, {length}) => Math.max(a, length), 0);

console.log(max);

Get array with the longest length

In this case, you don't need to provide an initial value as it defaults to the first element of the array.

const arrays = [[1],[3,3,4],[3,4],[4,5],[6]];

const max = arrays.reduce((a, v) => v.length > a.length ? v : a);

console.log(max);

user_35589600

In the first code, the reduce function is returning a number, while in the second, it is returning an array. When you're returning a number, the next time your function is called, it takes the length of a number, not an array. The second code is correct, because you're taking the length of an array every time.

user_133765110
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