aoc-5.js

var fs = require('fs');
var text = fs.readFileSync('input.txt', 'utf8');
var data = text.split('\n').map(Number);

function count (data) {
    var counter = 0;

    for (var i = 0; i < data.length && i >= 0;) {
        var _i = i;
        
        counter += 1;
        i += data[i];

        data[_i] = data[_i] + (data[_i] >= 3 ? -1 : 1);
    }

    return counter;
}

console.log(count(data));