JavaScript Problem: A Tricky JavaScript Interview Question

In this tutorial we are going to look at a JavaScript problem that is sometimes used to weed out JS developers during a JavaScript interview. This problem shows the interplay of scope and closure.

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Tutorials referred to in this video:

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[Hooghog]

This video is such a superb example of improving deep understanding of javascript every step of the way. Much appreciated!

[ChrisTian-oxnr]

Such a great video once again! Thank you!!

[batchrocketproject]

I paused at the start and correctly predicted the unwanted behaviour. However, being lazy I ran the routine by directly typing into the browser console rather than saving and loading a document. I became frustrated as the result was 6, just 6, nothing else (not even the string part of the template literal). if I used the up arrow and re-ran the routine in console, the 6 incremented by 5 each time. When I ran it from a document it behaved as expected. I'm still scratching my head to work out why it was returning an integer when typed directly (any insight greatly appreciated).

The use of let in the outer loop to create closure is clever (and I had not predicted that but will not forget it!). My only other refinement, a minor point, was to change the milliseconds interval to 2000*(1+i) to make the console logs sequentially over 5 two-second intervals, as I assumed that was the intention.

Excellent discussion, especially reasoning through the stages. Thanks. 💯

[zameeebasha]

We can also get the expected result using bind:


const arr = [5, 10, 15, 20, 25];

for (var i=0; i<arr.length; i++) {
setTimeout(function(i){
console.log(`Index: ${i}, element: ${arr[i]}`)
}.bind(null, i), 2000);
}

[arunkaiser]

Pls do video series on "Tricky Javascript Interview Questions" this topic

[Chantaletecker]

Also works with a third argument of setTimeout function

[chamnil]

Thank you SIR, i got it now, what's with the third argument in the setTimeout function?

[uthirapathikuppusamy]

I used let for variable declaration the loop completely working after 2s

[KatieGeorgieva]

What if the task is to show each of the results every 2 seconds? Why not use sync/await?

[wilsonfaustino]

I do not get why it still does not work even if I change 2000 to zero inside the set time out.