JavaScript Closures, Callbacks and Callback Hell, Currying.

JavaScript Closures, Callbacks and Callback Hell, Currying Function.

1. JavaScript Closures.

   Closures is the combination of a bundled function.

   Closures gives you access to an outer function's scope from an inner function.

function init() {

    var name = 'Mozilla';

    function displayName() {

        console.log('name --->', name);

        // output : name ---> Mozilla

    }

    displayName();

}

init();

2. JavaScript Callbacks.

   Function passed as an argument.

   

function test1(name, callback) {

    console.log('name---->', name);    

    callback();    

}

function test2() {

    console.log('test2---->');    

}

test1("Ankit", test2);

//output :

// name----> Ankit

// test2---->

3. JavaScript Callback Hell.

   Every callback depends / waits for the previous callback.

setTimeout(() => {

    console.log('test 1--->');

    setTimeout(() => {

        console.log('test 2--->');

    }, 2000);    

}, 5000);

// output :

// test 1--->

// test 2--->

4. JavaScript Currying Function.

function sendEmail(to) {

    return function (from) {

        return function (body) {            

            return to + ' - ' + from + ' - ' + body;

        }

    }

}

let to = sendEmail('to@gmail.com');

let from = to('from@gmail.com');

console.log(from('Body data'));

// output : to@gmail.com - from@gmail.com - Body data.

function sendEmail(to) {

    return function (from) {

        return function (body) {            

            return to + ' - ' + from + ' - ' + body;

        }

    }

}

console.log(sendEmail('to@gmail.com')('from@gmail.com')('Body data.'));

// output : to@gmail.com - from@gmail.com - Body data.

//using arrow function

const sendemail = (to) => (from) => (body) => { return to + ' - ' + from + ' - ' + body};

console.log('sendemail--->', sendemail('to@gmail.com')('from@gmail.com')('Body data.'));

// output : sendemail---> to@gmail.com - from@gmail.com - Body data.