var arr=[];
let A = ()=>{
    //
     return B(function(){
            
            return C(function(){
                
                 let a = 10;
                 arr.push(a);
                 return arr
            })
            
    
    })

}

1. Change asynchronism into synchronous form with await
2. Manually encapsulate the returned logic in then with promise
3. Use callback to pass the returned logic to the inner function through callback

learn about Promise

 let A =new Promise((resolve,reject) => {
    function B(function(){
       resolve();
    })
}).then(()=>{
function C(function(){
                
    let a = 10;
    arr.push(a);
            
  })
})