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Automatically check a checkbox with a userscript?

Some sites (namely Steam Community Market) require the user to manually check a specific checkbox for repetitive actions like buying items.

I’d like to have that checkbox always checked.

  • URL:
  • element:
    <input id="market_buynow_dialog_accept_ssa" type="checkbox" value="0" name="accept_ssa">

Can that be done with Tampermonkey?

I found document.getElementById("checkbox").checked = true; which seems logical to me. I put it in a new Tampermonkey script and added steam market to the list of websites the script activates on but it didn’t work.



  1. Find a pattern in the urls where the userscript should be executed, for example if it’s then we can assume the part starting with 730 is volatile so we’ll replace it with * in @include key.
  2. We should wait for the checkbox element to be added on the page, there are two methods: MutationObserver-based (I’ll use setMutationHandler wrapper) and setInterval-based (best known wrapper is waitForKeyElements). Both are plugged in via @require key.

// ==UserScript==
// @name        Steam - accept the agreement
// @include*
// @require
// ==/UserScript==

// maybe the elements are already on the page

// but anyway set a MutationObserver handler for them
setMutationHandler(document, 'input[type="checkbox"]', checkThem);

function checkThem(nodes) {
    nodes.forEach(function(n) { n.checked = true });

More info: Greasespot wiki.