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Claim AutoFill: Settings fields programatically

Experiment #21216th July, 2021by Joshua Nussbaum

After my last experiment create an autofill extension, I wanted to to figure out how to set different input fields programatically.

Most are easy, but setting <input type="file"/> was a little harder.

Text field

Setting a text field is straight forward and it works similarly for most field types, like number, tel, etc…

document.querySelector(`input[type=text][name=${fieldName}]`).value = newValue

Select field

Works similar to a text field, except the value maps to the value set by <option value="...">...</option>

document.querySelector(`select[name=${fieldName}]`).value = newValue


Similar to other inputs, except instead of setting the .value attribute, it uses .checked:

document.querySelector(`input[type=checkbox][name=${fieldName}]`).checked = boolean 

Redio button

Similar to checkbox, except there are multiple inputs with the same name, so the selector requires the value as well:

document.querySelector(`input[name=${fieldName}][value=${value}]`).checked = true


This one was a bit tricky. It required converting to files to base64 URLs and using DataTransfer objects.

So say I have a base64 image URL, like this:


To load it into the <input type="file"/>:

// split on comma to remove the first part "data:image/png;base64,"
const fileEncoded = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAlcAAACVC.....'.split(',')[1]
// set a filename
const fileName = 'foo.png'

// create a data transfer object
const dt = new DataTransfer()
// add file object with name
dt.items.add(new File([fileEncoded], fileName))

// get the input
const input = document.querySelector(`input[type=file][name=${fieldName}]`)

// set the files using the data transfer object
input.files = dt.files

Tricky, but it works!

Submitting the form

Use dispatchEvent() to submit the form:

document.querySelector('form').dispatchEvent(new Event('submit'))
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