Submitting form data

I'm trying to create a bookmarklet in chrome that will modify a form's input data.

One of the fields looks like this:
<input type="text" id="shipping:firstname" name="shipping[firstname]" value="John" title="First Name" maxlength="255" class="input-text required-entry" placeholder="First Name*" onchange="shipping.setSameAsBilling(false)" autocapitalize="off" autocorrect="off" oldautocomplete="remove" autocomplete="off">

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Yet when I click my bookmark, it just pops up the new user's name in a blank page...
code is simply:
javascript:  let first = document.getElementById('shipping:firstname');  first.value='John';

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What am I doing wrong?
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Dustin SaundersDirector of OperationsCommented:
Try writing it like this:

javascript:(function(){ let first = document.getElementById('shipping:firstname'); first.value = 'John';}(window.document));

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sirbountyAuthor Commented:
Thanks for the quick turnaround!
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