javascript - Compacting multiple promises -

i have following javascript promise chain. works expected.

signup (data) {   return onefunction(username).then((usernameexist) => {     return firebaseauth.createuserwithemailandpassword(data.email, data.password).then((user) => {       firebasedb.ref('users/' + user.uid + '/public/').set(userdata).then()       utils.updateusernamemapping(data.username, user.uid).then()       return user.updateprofile({         displayname: data.displayname       }).then(function () {         return user       }, error => {          throw error       })     })   }).catch(error => {     throw error   }) } 

however, believe signup function hard decipher because of nested levels. tried change following approach:

userpromise .then() .then() .then(); 

but couldn't working because user variable needs passed down chain. ideally, minimize code readability , use 1 catch() efficiency. ideas appreciated.

update: following feedback bergi, below updated code:

signup (email, password, displayname, username) {   const userdata = { username: username, lastlogin: firebase.database.servervalue.timestamp }   return utils.checkifuserexists(username).then(usernameexist => {     return firebaseauth.createuserwithemailandpassword(email, password)   }).then(user => {     return promise.all([       firebasedb.ref('users/' + user.uid + '/public/').set(userdata),       utils.updateusernamemapping(username, user.uid),       user.updateprofile({displayname})     ]).then(() => user)   }) }, 

error handlers rethrow error pointless, omit them.

you can unnest outermost level usernameexist variable don't need anywhere else:

signup (data) {   return onefunction(username).then(usernameexist => {     return firebaseauth.createuserwithemailandpassword(email, password);   }).then(user => {     return promise.all([       firebasedb.ref('users/' + user.uid + '/public/').set(userdata),       utils.updateusernamemapping(username, user.uid),       user.updateprofile({displayname})     ]).then(() => user);   }); } 

there's nothing wrong nested then ensures user returned in end. there a few approaches tackle problem, nesting closures fine.