Creates a site scoped scriptlink custom action

CreateScriptLinkCA.js

var siteurl = _spPageContextInfo.webAbsoluteUrl;
var formDigest;
var generateCustomAction = function(location,sequence,title,description,url){
	location = location || 'ScriptLink';
	sequence = sequence || 1000;
	title = title || 'Test Custom Action';
	description = description || 'Test Description';
	url = url || '~sitecollection/Style Library/scripts/jquery-3.3.1.js';
	return JSON.stringify({ 
		'__metadata': { 
			'type': 'SP.UserCustomAction' 
		}, 
		'Location':location.toString(), 
		'Sequence':sequence.toString(), 
		'Name':title.toString(),
		'Description':description.toString(), 
		'ScriptSrc': url.toString()
	});
}
var getFormDigest = function(){
	return $.ajax({
		type: 'Post',
		url: siteurl + '/_api/contextinfo',
		dataType: 'json',
		headers: {
			'Accept': 'application/json;odata=verbose'
		}
	}).then(function(data){
		return data.d.GetContextWebInformation.FormDigestValue;
	});
};
var set_formDigest = (function(callback){callback = callback || function(){console.log('FormDigest Set');};getFormDigest().then(function(digest){formDigest = digest;callback();})})();
var addSiteCustomAction = function(){
	$.ajax({
		url: siteurl + "/_api/site/UserCustomActions",
		headers: {
			"Accept": "application/json; odata=verbose",
			"Content-Type": "application/json; odata=verbose",
			"X-RequestDigest": formDigest
		},
		method: "POST",
		data: generateCustomAction(),
		success: function(data){
			console.log(data);
			if(data.d.results){
				console.log(data.d.results);
			}
		},
		error: function(data){
			console.log(data);
			console.error("Error: " + data);
		}
	});
};