Javascript : Callback using Closure an Example

create database folder.add three files sagar.json,sangram.json,sachin.json.

sagar.json

{

"secret":"He punched his brother when mother is not around"

}

sangram.json

{

"secret":"He is running around to fix the problem"

}

sachin.json

{

"secret":"He eats rest of ice cream"

}

closure.js

Code:

"use strict";

const fs = require("fs");

let users = [

{

username: "sagar",

password: "ssd1778",

},

{

username: "sangram",

password: "ssd1110",

},

{

username: "sachin",

password: "ssd2183",

},

];

let printResult = function (username, secret) {

console.log(`USERNAME:${username},SECRET:${secret}`);

};

let callback = function (username) {

return function (err, response) {

if (err) {

throw new Error(err);

}

let data = JSON.parse(response);

let secret = data["secret"];

printResult(username, secret);

};

};

let getSecret = function (users) {

users.forEach((obj) => {

fs.readFile("./database/" + obj.username + ".json", "utf8",

callback(obj.username));

});

};

getSecret(users);

Output:

USERNAME:sagar,SECRET:He punched his brother when mother is not around

USERNAME:sangram,SECRET:He is running around to fix the problem

USERNAME:sachin,SECRET:He eats rest of ice cream

Explanation:

'callback()' function returns function,it's inner function remember value of

variable 'username' which is function parameter of 'callback()' function.

then this inner function 'printResult()' print results.