3 examples of 'synchronous ajax' in JavaScript

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function doAjax(method, url, callback) {
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    if (window.GM_xmlhttpRequest) {
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        GM_xmlhttpRequest({
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            method: method,
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            url: url,
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            onload: callback,
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        });
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    } else {
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        var xhr = new XMLHttpRequest();
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        xhr.onreadystatechange = function() {
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            if (xhr.readyState !== 4 || xhr.status !== 200) return;
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            callback(xhr);
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        };
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        xhr.open(method, url, true);
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    }
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}

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function ajax(options, callback) {
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	var origin = window.location.protocol + "//" + window.location.hostname + ":" + exports.PORT;
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	var request = $.ajax({
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		type: "GET",
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		url: origin + options.endpoint,
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		data: {
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			data: JSON.stringify(options.data),
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			socketId: options.connection.getSocketId()
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		},
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		async: options.async,
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		cache: false
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	});
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	if (options.async) {
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		request.done(function() {
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			if (request.status !== 200) throw new Error("Invalid status: " + request.status);
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			return callback(null, request.responseText);
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		});
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		request.fail(function(_, errorText) {
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			if (request.status !== 200) throw new Error("Invalid status: " + request.status);
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			throw new Error(errorText);
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		});
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	}
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	else {
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		if (request.status !== 200) throw new Error("Invalid status: " + request.status);
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		else return request.responseText;
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	}
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}

2
function createAjax() {
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	let httpRequest;
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	if (window.XMLHTTPRequest) {
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		httpRequest = new XMLHttpRequest();
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	}else if(window.ActiveXObject){
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		httpRequest = new ActiveXObject('Microsoft.XMLHTTP');
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	}
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	return httpRequest;
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}