JSFiddle - React, Tailwind, and code Playground

JavaScript

var array = [1,1,1,1,2,3,3,3,4,5,6];

function streak(arr) {
    var i,
        temp,
        streak = 1,
        maxStreak = 0,
        prevNumber,
        length = arr.length;

    for(i=1; i<length; i++) {
        prevNumber = arr[i-1];
        if (arr[i] == prevNumber) {
            streak += 1;
        } else {
            if(streak > maxStreak) {
                maxStreak = streak;
                streak = 1;
            }
        }
    }
    return maxStreak;
}

var streak = streak(array);
console.log(streak);