Time, tide and Javascript wait for none.
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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.
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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
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Reason?
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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
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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
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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();
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