Fetch API

// Складываем два числа удаленно
let resultA, resultB, resultC;

// ES6 подтягивает API, который возвращает промис.
function addAsync(num1, num2) {
    
    return fetch('http://mysite?num1='+num1+'&num2='+num2)
            
            .then(x => x.json()
    );
}

addAsync(1, 2)
    .then(success => {
        resultA = success;
        return resultA;
    })

    .then(success => addAsync(success, 3))

    .then(success => {
        resultB = success;
        return resultB;
    })

    .then(success => addAsync(success, 4))
    .then(success => {
        resultC = success;
        return resultC;
    })
    .then(success => {
        console.log('total: ' + success)
        console.log(resultA, resultB, resultC)
    });

index.php :

echo json_encode([$_REQUEST['num1']+$_REQUEST['num2']]);
die();

Вариант для POST:

// Складываем два числа удаленно
let resultA, resultB, resultC;

function addAsync(num1, num2) {
	
	const payload = {
		 num1: num1, 
		 num2: num2
    }

	return fetch('http://test', {
			method: "POST",
            body: new URLSearchParams(payload),
			headers: new Headers({
			  'Content-type': 'application/x-www-form-urlencoded; charset=UTF-8'
			})
        })
     	   
        .then(x => x.json()
	);
}

addAsync(1, 2)
    .then(success => {
        resultA = success;
        return resultA;
    })

    .then(success => addAsync(success, 3))

    .then(success => {
        resultB = success;
        return resultB;
    })

    .then(success => addAsync(success, 4))
    .then(success => {
        resultC = success;
        return resultC;
    })
    .then(success => {
        console.log('total: ' + success)
        console.log(resultA, resultB, resultC)
    });

Версия Fetch API Для отправки JSON на сервер:

let Likes = class Likes {
    static toggle(element, event) {
        event.preventDefault();
        let post_id = element.dataset.post_id
        let response = request('/postlike/' + post_id, 'POST', {})

        response
            .then(data => {
                if (data.status === 'ok') {
                    console.log(data)
                } else if (data.status === 'error') {
                    console.log(data)
                } else {
                    console.log(data)
                }
            })
    }
};

//<meta name="_token" content="{{ csrf_token() }}">

function request(url, method, payload) {
    return fetch(url, {
        method: method,
        body: JSON.stringify(payload),
        headers: new Headers({
            'Accept': 'application/json',
            'Content-type': 'application/json',
            'X-CSRF-Token': document.querySelector("meta[name='_token']").getAttribute('content')
        })
    })
        .then(r => r.json())
        .catch(error => console.error(error))
}