Episode 11 : 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();
```

Watch Live On Youtube below:

Namaste Javascript

Episode 11 : setTimeout + Closures Interview Question