Automatically check a checkbox with a userscript?

Question

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:
  http://steamcommunity.com/market/listings/730/USP-S%20%7C%20Torque%20(Field-Tested)
• 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.

Answer 1

1. Find a pattern in the urls where the userscript should be executed, for example if it's http://steamcommunity.com/market/listings/730/USP-S%20%7C%20Torque%20(Field-Tested) 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     http://steamcommunity.com/market/listings/*
// @require     https://greasyfork.org/scripts/12228/code/setMutationHandler.js
// ==/UserScript==


// maybe the elements are already on the page
checkThem([].slice.call(document.querySelectorAll('input[type="checkbox"]')));

// 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.