SOURCE

let a = new Promise((resolve,reject)=>{
    resolve()
})

a.then(res=>{
    console.log('a')
    a.then(res=>{
    console.log('c')
})
})
    a.then(res=>{
    console.log('b')
})

function add(getX,getY,cb){
    console.log(getX)
    let x,y
    getX(function(xValue){
        x = xValue
        if(y != undefined){
            db(x+y)
        }
    })
    getY(function(yValue){
        y = yValue
        if(x != undefined){
            db(x+y)
        }
    })
}
let d = function(item){
   
}   
let b = function(item){
    
}
add(d,b,function(sum){
    console.log(sum)
})