Notes.md

setTimeout + Closures Interview Question :

Time, tide and Javascript wait for none.

function x() {
    var i = 1;
    setTimeout(function () {
        console.log(i);
    }, 3000);
    console.log("Namaste Javascript");
}
x();
// Output:
// Namaste Javascript
// 1 // after waiting 3 seconds

• We expect JS to wait 3 sec, print 1 and then go down and print the string. But JS prints string immediately, waits 3 sec and then prints 1.
• The function inside setTimeout forms a closure (remembers reference to i). So wherever function goes it carries this ref along with it.
• setTimeout takes this callback function & attaches timer of 3000ms and stores it. Goes to next line without waiting and prints string.
• After 3000ms runs out, JS takes function, puts it into call stack and runs it.
• Q: Print 1 after 1 sec, 2 after 2 sec till 5 : Tricky interview question

We assume this has a simple approach as below :

function x() {
    for (var i = 1; i <= 5; i++) {
        setTimeout(function () {
            console.log(i);
        }, i * 1000);
    }
    console.log("Namaste Javascript");
}
x();
// Output:
// Namaste Javascript
// 6
// 6
// 6
// 6
// 6

Reason?

• This happens because of closures. When setTimeout stores the function somewhere and attaches timer to it, the function remembers its reference to i, not value of i.

• All 5 copies of function point to same reference of i. JS stores these 5 functions, prints string and then comes back to the functions. By then the timer has run fully. And due to looping, the i value became 6. And when the callback fun runs the variable i = 6. So same 6 is printed in each log

• To avoid this, we can use let instead of var as let has Block scope. For each iteration, the i is a new variable altogether(new copy of i).

• Everytime setTimeout is run, the inside function forms closure with new variable i

But what if interviewer ask us to implement using var?

function x() {
    for (var i = 1; i <= 5; i++) {
        function close(i) {
            setTimeout(function () {
                console.log(i);
            }, i * 1000);
            // put the setT function inside new function close()
        }
        close(i); // everytime you call close(i) it creates new copy of i. Only this time, it is with var itself!
    }
    console.log("Namaste Javascript");
}
x();