javascript - How to execute the callback at the end of the function calling it? -

in node js application have function reads line line file, stores contents of file in array , calls callback function array passed callback.

function readfile(callback) {     var fs = require('fs'), readline = require('readline');      var rd = readline.createinterface({         input: fs.createreadstream('./file.txt'),         output: process.stdout,         terminal: false     });      var data = [];      rd.on('line', function(line) {         data.push(line);     });      callback(data); } 

the problem i'm facing readfile running callback before file read , data array filled. callback running empty array. how can make run once array filled? thanks.

readline has close event fired on "end"

so, code should be

function readfile(callback) {     var fs = require('fs'), readline = require('readline');      var rd = readline.createinterface({         input: fs.createreadstream('./file.txt'),         output: process.stdout,         terminal: false     });      var data = [];      rd.on('line', function(line) {         data.push(line);     }).on('close', function () {         callback(data);     }); } 

in es2015:

function readfile(callback) {     var fs = require('fs'), readline = require('readline');      var rd = readline.createinterface({         input: fs.createreadstream('./file.txt'),         output: process.stdout,         terminal: false     });      var data = [];      rd.on('line', line => data.push(line)).on('close', () => callback(data)); } 

or even

function readfile(callback) {     var fs = require('fs'),          readline = require('readline'),         data = [];      readline.createinterface({         input: fs.createreadstream('./file.txt'),         output: process.stdout,         terminal: false     })     .on('line', line => data.push(line))     .on('close', () => callback(data)); } 

but i'm not sure of 2 es2015 codes more readable