Gadget-editButton.js

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$( function () {
	var namespace = mw.config.get('wgNamespaceNumber');
	
	var wikiEdit = document.createElement("a");
	wikiEdit.setAttribute("id", "wikiEdit");
	wikiEdit.setAttribute("class", "button");
	
	if (namespace === 14 || namespace === 3004 || namespace == 2 || namespace == 0)
		wikiEdit.setAttribute("href", "/wiki/index.php?title=" + mw.config.get('wgPageName') + "&veaction=edit")
	else
		wikiEdit.setAttribute("href", "/wiki/index.php?title=" + mw.config.get('wgPageName') + "&action=edit")
		
	var wikiEditButton = document.createElement("button");
	wikiEdit.appendChild(wikiEditButton);
	
	var wikiEditButtonIcon = document.createElement("i");
	if (namespace === 14 || namespace === 3004 || namespace == 2 || namespace == 0)
		wikiEditButtonIcon.setAttribute("class", "fas fa-pencil");
	else 
		wikiEditButtonIcon.setAttribute("class", "fas fa-code");
	wikiEditButton.appendChild(wikiEditButtonIcon);
	
	var wikiEditButtonLabel = document.createElement("span");
	if (namespace === 14 || namespace === 3004 || namespace == 2 || namespace == 0)
		wikiEditButtonLabel.innerText = " Edit";
	else 
		wikiEditButtonLabel.innerText = " Edit source";
	wikiEditButton.appendChild(wikiEditButtonLabel);
	
	document.getElementById("wikiActions").before(wikiEdit);
})

MediaWiki

Gadget-editButton.js