you can use co + generator, but it is not recommended.

it is best to solve the problem of gulp compilation, but it is not possible to rewrite

var testAjax = async function () {
    var promise = Promise.resolve()
    for(let i = 0; i < 5; iPP) {
        promise = promise.then(function(){
            console.log('test1: ' + i);
            let url = "xxx";
            return $.ajax({
                url: url,
                dataType: 'json',
                type: "GET",
                //async: false
            }).then(function() {
                console.log('test2: ' + i);
            })
        })
    }
}
testAjax();

use async.await to meet your needs. Just write the request differently:

var testAjax = function () {
    for(let i = 0; i < 5; iPP) {
    // i
        (function (i) {
            console.log('test1: ' + i);
            var url = "xxx";
              $.ajax({
                url: url,
                dataType: 'json',
                type: "GET",
                //async: false
            }).then(function() {
                console.log('test2: ' + i);
            }).fail(function() {
                console.log('error')
            });
        })(i)
    }
}
testAjax();