How do I change the text of a span element using JavaScript?

If I have a span, say:

<span id="myspan"> hereismytext </span>

How do I use JavaScript to change “hereismytext” to “newtext”?

For modern browsers you should use:

document.getElementById("myspan").textContent="newtext";

While older browsers may not know textContent, it is not recommended to use innerHTML as it introduces an XSS vulnerability when the new text is user input (see other answers below for a more detailed discussion):

//POSSIBLY INSECURE IF NEWTEXT BECOMES A VARIABLE!!
document.getElementById("myspan").innerHTML="newtext";