how do i combine a string and an int and find its value in an array?

var tribonacci = function(n) {
    let arr = [0, 1, 1];
    for (i = 2; i < n; i++) {
        arr.push(arr.slice(-3).reduce((a,b) => a+b));
    }
    return arr[n];
};
console.log(tribonacci(1))
console.log(tribonacci(2))
console.log(tribonacci(3))
console.log(tribonacci(4))
console.log(tribonacci(7))
console.log(tribonacci(14))