How can I make the same function execute with javascript promises? -

i want execute same function 3 times using javascript promises. each time function called text file read line line , answer each line written text file. want javascript promise wait till previous function done, reason runs 3 functions @ once, thereby writing 3 files @ once. since i'm processing massive file, writing 3 text files @ once takes long time.

can please me figure out how run correctly? i'm new promises, , need can get.

here code:

function verifytransactions(filename,functionname,obj,depth){     var rd = readline.createinterface({       input: fs.createreadstream('../paymo_input/stream_payment.csv'),       output: process.stdout,       terminal: false     });     rd.on('line', function(line) {       var userdata = extractuserdata(line)       if (userdata !== undefined){         var id1 = userdata[0], id2 = userdata[1];         if (obj[id1]){           console.log(id1)           fs.appendfilesync('../paymo_output/'+filename     +'.txt',functionname(obj,id1,id2,depth)+"\n", "utf-8",{'flags': 'a'});         }         else{           console.log("nope")           fs.appendfilesync('../paymo_output/'+filename+'.txt', "unverified"+"\n", "utf-8",{'flags': 'a'});         }       }     });     rd.on('end',function(){       console.log("on nexxttttt")     }) }  promise.resolve("output1")   .then(function(file){     verifytransactions(file,pasttransaction,usertransactions);     console.log("writing file 2 soon")     return "output2";})   .then(function(file){     verifytransactions(file,breadthfirstsearch,usertransactions,2);     return "output3";})   .then(function(file){     verifytransactions(file,breadthfirstsearch,usertransactions,4);     return "finito!!!";})   .then(function(file){     console.log(file);   }) 

if want use promises wait until function has finished job, you'll need 3 things:

1. return unresolved promise object function
2. inside function, resolve promise once decide function has finished job
3. wherever call function, need wait promise resolve before doing more things. that's accomplished .then().

in other words:

function myfunction(input) {    var promise = new promise();     /* things, then: */    someeventemitter.on('myevent', function() {      promise.resolve(returnvalue); // done!    });     return promise; }  myfunction(input1)   .then((function(result) { // run function once promise has resolved     console.log('myfunction returned: ' + result);    }); 

if want use promises several asynchronous things in sequence, you'll need what's called promise chaining:

myfunction(input1) // returns promise, has .then() method   .then(function(result) {     console.log('first result: ' + result);     return myfunction(input2); // return new promise inside of function passed .then()   }) // new promise has .then() method of own   .then(function(result2) {     console.log('second result: ' + result2);     return myfunction(input3);   })   .then(function(result3) {     console.log('third result: ' + result3);   });