javascript - Difference between callback hell and nested promises -

i started working nodejs , mongodb(using monk). when came across term "callback hell". in code doing same thing. example-

dbcall(d1, function(e, docs){  if(docs.length!=0)   dbcall(d2, function(e, docs1){    if(docs1.length!=0)     dbcall(d3, function(e, docs2){       //doing docs,docs1,docs2     })    })  })  

this when started reading on "promises", , came across article - https://strongloop.com/strongblog/promises-in-node-js-with-q-an-alternative-to-callbacks/

since needed both docs , docs1 inside third callback used nested promises.

dbcall(d1) .then(function(docs){   if(docs.length!=0)    return dbcall(d2)    .then(function(docs1){      if(docs1.length!=0)       return dbcall(d3)       .then(function(docs2){         //doing docs,docs1,docs2       })     })   }) 

from above code snippet have following questions(/doubts) :

1. apart making code more readable, promises have performance benefits?
2. nesting promises , callback hell looks similar me. there difference?

i new concept of promises. appreciated.

apart making code more readable, promises have performance benefits?

probably not.

nesting promises , callback hell looks similar me. there difference?

promises don't automatically prevent callback hell. because more flexible "simple" callbacks, can composed in different ways, makes easier avoid callback hell.

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since needed both docs , docs1 inside third callback used nested promises.

sometimes nested promises unavoidable. however, there no need nest them if don't have executed sequentially. execute them in parallel:

promise.all([  dbcall(d1),  dbcall(d2),  dbcall(d3) ]).then(function(docs) {   // docs array: [docs, docs1, docs2] });