# How to find and return the longest repeating series of numbers in array with JavaScript

We are required to write a JavaScript function that takes in an array of Numbers that may contain some repeating elements. The function should return the length of the longest repeating number sequence from the array.

For example −

If the input array is −

const arr = [2, 1, 1, 2, 3, 3, 2, 2, 2, 1];

Then the output should be 3 because the number 2 is repeated 3 times consecutively in the array (and that's the highest number).

## Example

const arr = [2, 1, 1, 2, 3, 3, 2, 2, 2, 1];
const findLongestSequence = (arr = []) => {
const res = arr.reduce((acc,val,ind) => {
if(acc.length && acc[acc.length-1][0] === val){
acc[acc.length-1].push(val);
}else{
acc.push([val]);
};
return acc;
},[]).reduce((acc, val) => {
return val.length > acc.length ? val : acc;
}); return res.length;
}
console.log(findLongestSequence(arr));

## Output

And the output in the console will be −

3

Updated on: 21-Nov-2020

395 Views