Get value of Span Text

The accepted answer is close... but no cigar!

Use textContent instead of innerHTML if you strictly want a string to be returned to you.

innerHTML can have the side effect of giving you a node element if there's other dom elements in there. textContent will guard against this possibility.

<script type="text/javascript">
document.getElementById('button1').onChange = function () {
    document.getElementById('hidden_field_id').value = document.getElementById('span_id').innerHTML;
}
</script>

var span_Text = document.getElementById("span_Id").innerText;

console.log(span_Text)

<span id="span_Id">I am the Text </span>