Gadget-editButton.js

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$( function () {
	var wikiEdit = document.createElement("a");
	wikiEdit.setAttribute("id", "wikiEdit");
	wikiEdit.setAttribute("class", "button");
	
	var wikiEditButton = document.createElement("button");
	wikiEdit.appendChild(wikiEditButton);
	
	var wikiEditButtonIcon = document.createElement("i");
	wikiEditButtonIcon.setAttribute("class", "fas fa-pencil");
	wikiEditButton.appendChild(wikiEditButtonIcon);
	
	var wikiEditButtonLabel = document.createElement("span");
	wikiEditButtonLabel.innerText = " Edit";
	wikiEditButton.appendChild(wikiEditButtonLabel);
	
	document.getElementById("wikiActions").before(wikiEdit);
})

MediaWiki

Gadget-editButton.js