Edit in JSFiddle

let total_A, total_B, total_C;

function addAsync(number_1, number_2) {
    return Promise.resolve(number_1 + number_2);
}

addAsync(1, 2)
.then(success => {
    total_A = success;
    return total_A;
})
.then(success => addAsync(success, 3))
.then(success => {
    total_B = success;
    return total_B;
})
.then(success => addAsync(success, 4))
.then(success => {
    total_C = success;
    return total_C;
})
.then(success => {
    console.log('Tổng của 1 + 2 + 3 + 4 = ' + success);
});