SOURCE

const str = 'abc12133321cab'

function getRe(str) {
    const arr = str.split('')
    let maxLen = 0
    for(let i = 0; i <arr.length; i++) {

        for(let k = i + 1; k < arr.length; k++) {
            if (arr[i] === arr[k]) {
                let flag = true
                for(let n = 0; n < k - n; n++) {
                    if (i + n <= k - n) {
                        if (arr[i + n] !== arr[k - n]) {
                            flag = false
                            break
                        }
                    }
                }
                if (flag) {
                    if (maxLen < k - i + 1) {
                        maxLen = k - i + 1
                    }
                }
            }

        }
    }

    return maxLen
}

console.log(getRe(str))