SOURCE

function test(arr) {
    const arr2 = [];
    let temp = [];
    let temp2 = [];
    arr.forEach((a, i) => {
        const conf1 = a - arr[i - 1] === 1
        const conf2 = arr[i + 1] - a === 1
        if (i > 0) {
            if (conf1) {
                if (temp.indexOf(arr[i - 1]) < 0) {
                    temp.push(arr[i - 1])
                }
                if (temp.indexOf(a) < 0) {
                    temp.push(a)
                }
            } else {
                temp.length && arr2.push(temp)
                !conf2 && arr2.push([a])
                temp = []
            }
        }

    })
    temp.length && arr2.push(temp)
    return arr2.map(v => {
        const t = v.length > 1 && v.splice(1, v.length - 2, '->')
        return v.join('');
    })

}
var result = test([1, 2, 3, 4, 6, 7, 9, 13, 15])
console.log(result)
console.log(test([2, 3, 4, 7, 8, 22, 55, 56, 77, 88, 99, 100]))