callback.js

//syncronour programming is executed line by line;


console.log("1");
console.log("2");
setTimeout(()=>{
    console.log("this will take time");//asyncronous fn;
},7000);

console.log("3");
console.log("4");


//callback fn are the fn that are transferred to other fn as an argment;

function sum(a,b){
    console.log(a+b);
}

function call(a,b,sum2){
    sum2(a,b);
}

call(5,6,sum);//parentheses are not written



//callback hell-. nested calback problem

function prob(dataid,next){
    setTimeout(()=>{
        console.log(dataid);
        if(next){
            next();
        }
    },2000);
}

//prob(1);
//prob(2);
//prob(3);these three will be printed all together after 6s but we wanted one by one;

//callback hell---pyramid structure,,difficult to understand;
prob(1,()=>{
    prob(2,()=>{
        prob(3);
    });
})