User:Þjarkur/CleanDiffURLs.js

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This user script seems to have a documentation page at User:Þjarkur/CleanDiffURLs.

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/*
  Gives clean diff URLs
  Turns this: /w/index.php?title=Main_Page&diff=864711889&oldid=864629150&diffmode=source
  Into this: /wiki/Special:Diff/864629150/864711889
*/
(function () {
  setTimeout(function () { // Timeout to remove "&diffmode=source" which is added later
    var url = new URL(window.location.href)
    var oldid = url.searchParams.get("oldid")
    var diff = url.searchParams.get("diff")
    var hash = window.location.hash.substr(1) ? '#'+window.location.hash.substr(1) : ''
    var output

    if (oldid && !diff) {
      output = 'Special:Permalink/' + oldid
    } else if (oldid && diff === 'prev') {
      output = 'Special:Diff/' + oldid
    } else if (diff && oldid) {
      output = 'Special:Diff/' + oldid + '/' + diff
    } 

    if (output) {
      window.history.replaceState({}, '', '/wiki/' + output + hash)
    }
  }, 50)
})()