It always returns 0 because you compare it to min_val. Where min_val becomes the lowest value as 0 is lower then any value in the array. Therefor min_val will always be set equal to itself and stay 0. Here is my fix :

let ar =[14,99,10,18,19,11,34]
let min_val = Math.min(...ar)
console.log(min_val)

Math.min() receivec arguments like Math.min(-2, -3, -1) or with array Math.min(...arr). You are giving these arguments Math.min([14,99, 10, 18,19, 11, 34], 0) where 0 is minimum. So you are receiving 0. Also, no need for a loop as Math.min() finds the minimum. So, the solution would be:

    function values(ar) {
        var min_val = Math.min(...ar);
        //document.write(min_val);
        return min_val
    }
    console.log(values([14,99, 10, 18,19, 11, 34]))

or (to match your case)

function values(ar) {
    var min_val = Math.min(...ar);
    
    document.write(min_val);
}

in shortest form:

document.write(Math.min(...ar));

For your expected behavior, in order to find the minimum, you want to start at the absolute maximum (Infinity).

    var min_val = Infinity;
    for(var i = 0; i < ar.length; i++) {
         min_val = Math.min(min_val,ar[i]);
            }
document.write(min_val);
}
values([14,99, 10, 18,19, 11, 34]);```

This will write Infinity for empty arrays, or the minimum value for the input.