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Asynchronous JS

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📲Learn about Asynchronous JS Callback(Custom)📞

A callback is a function passed as an argument to another function, which is executed after the main function finishes it's job.

A callback is like a message you send to a friend saying,"Call me back when you're done with your work"
Basic callback function:

                        
function greet(name, callback) {
    console.log("Hello, " + name + "!");
    callback();
}

// This is the callback function
function sayGoodbye() {
    console.log("Goodbye!");
}

greet("Alice", sayGoodbye);

Here is another example.

Note:The setTimeout function in JS does not return a value from the function that it delays. Instead it returns a unique identifier.

                        
function Sum(num1,num2,callback){
    setTimeout(()=>{
        let result=num1+num2;
        callback(result);

    },2000);
}

Sum(4,5,function(result){
    console.log(result);
});

Here is another example.

                        
function Task1(callback){
    setTimeout(()=>{
        console.log("completed task 1");
        callback();

    },2000)
}

function Task2(callback){
    setTimeout(()=>{
        console.log("completed task 2");
        callback();
    },1500);
}

function Task3(callback){
    setTimeout(()=>{
        console.log("completed task 3");
        callback();
    },3000);
}

function Task4(callback){
    setTimeout(()=>{
        console.log("completed task 4");
        callback();
    },2500);
    
}


//Here output is Asynchronus. without callback.
/*
Task1();
Task2();
Task3();
Task4();
*/

Task1(()=>{
    Task2(()=>{
        Task3(()=>{
            Task4(()=>{
                console.log("All task is completed");
            });
        });

    });
});

🌐Application🈸

Form Validation


function formValidation(data,callback){
    let isvalid=callback(data);
    if(isvalid){
        console.log("Form is Valid");
    }
    else{
        console.log("Form is Invalid");
    }

}


formValidation({age:24},function(data){
    return data.age>=18;

});

formValidation("shanto@gmail.com",function(email){
    return email.includes("@");
});

Database Query


function fetchDataFromDatabase(query, callback) {
    // Simulate a database call
    setTimeout(() => {
        let data = { id: 1, name: 'Shanto Doe' }; // Example data
        callback(data);
    }, 1000);
}

// Custom callback to handle the data
fetchDataFromDatabase('SELECT * FROM users', function(data) {
    console.log('User name:', data.name);
});

fetchDataFromDatabase('SELECT * FROM users', function(data) {
    console.log('User ID:', data.id);
});