How to make executeQueryAsync() behave synchronously?

You can use JavaScript call backs or Promises/deferreds to work with JavaScript client object model

CallBack Example:

$(document).ready(function () {
        //don't exectute any jsom until sp.js file has loaded.        
        SP.SOD.executeFunc('sp.js', 'SP.ClientContext', prepareTables);
    });

function prepareTables() {
    getItemsWithCaml('External User Account Request',
            function (camlItems) {
                var listItemEnumerator = camlItems.getEnumerator();
                while (listItemEnumerator.moveNext()) {
                    var listItem = listItemEnumerator.get_current();
                    console.log(listItem.get_item('Title'));
                } console.log('Completed table prparation.');
            },
            function (sender, args) {
                console.log('An error occured while retrieving list items:' + args.get_message());
            });
}    

function getItemsWithCaml(listTitle, success, error) {
    var clientContext = new SP.ClientContext.get_current();
    var list = clientContext.get_web().get_lists().getByTitle(listTitle);
    var camlQuery = new SP.CamlQuery();
    var camlItems = list.getItems(camlQuery);
    clientContext.load(camlItems);
    clientContext.executeQueryAsync(
            function () {
                success(camlItems);
            },
            error
        );
};

Promises Example:

$(document).ready(function () {
        //don't exectute any jsom until sp.js file has loaded.        
        SP.SOD.executeFunc('sp.js', 'SP.ClientContext', prepareTables);
    });    

function prepareTables() {
    getItemsWithCaml('External User Account Request').then(
            function (camlItems) {
                var listItemEnumerator = camlItems.getEnumerator();
                while (listItemEnumerator.moveNext()) {
                    var listItem = listItemEnumerator.get_current();
                    console.log(listItem.get_item('Title'));
                } console.log('Completed table prparation.');
            },
            function (sender, args) {
                console.log('An error occured while retrieving list items:' + args.get_message());
            }
        );   
}    

function getItemsWithCaml(listTitle) {
    //use of $.Deferred in the executeQueryAsync delegate allows the consumer of this method to write 'syncronous like' code
    var deferred = $.Deferred();
    var clientContext = new SP.ClientContext.get_current();
    var list = clientContext.get_web().get_lists().getByTitle(listTitle);
    var camlQuery = new SP.CamlQuery();        
    var items = list.getItems(camlQuery);
    clientContext.load(items);
    clientContext.executeQueryAsync(
        Function.createDelegate(this,
            function () { deferred.resolve(items); }),
        Function.createDelegate(this,
            function (sender, args) { deferred.reject(sender, args); }));

    return deferred.promise();
};
Origin: http://www.sharepointnadeem.com/2014/10/sharepoint-using-deferredspromises-or.html