My function is not printing anything

Question

The algorithm is supposed to sum two elements of the array and insert them between the two elements used in the sum.

Example input ==> output :

[1, 2, 3, 4] ==> [ 1, 3, 2, 5, 3, 7, 4]    

function growArray() {
    var sequence = [];
    sequence = window.prompt("Enter a number sequence", "1 2 3 4");
    for (var i = 0; i < sequence.length - 1; i += 2) {
        if (!((i + 1) in sequence)) {
            return;
        } // prevent out of bounds access
        var new_value = sequence[i] + sequence[i + 1];
        sequence.splice(i + 1, 0, new_value); // insert value at i + 1
    }

    for (var v in sequence) {
        document.write(v + ", ");
    }
}

<p>
    <input id="f1" type="button" value="Function 1" onclick="growArray();" />Click to expand array
</p>

Answer 1

here is what gets the output

function growArray() {
    var input = window.prompt("Enter a number sequence", "1 2 3 4");
    var sequence = input.split(" ");
    for (var i = 0; i < sequence.length - 1; i += 2) {
        if (i + 1 > sequence.length) {
            return;
        } // prevent out of bounds access
        var new_value = parseInt(sequence[i]) + parseInt(sequence[i + 1]);
        sequence.splice(i + 1, 0, new_value); // insert value at i + 1
    }
    document.write(sequence.toString());
}

<p>
    <input id="f1" type="button" value="Function 1" onclick="growArray();" />Click to expand array
</p>