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Posted on • Originally published at learn.coderslang.com

JavaScript Interview Question #39: How does setTimeout work inside the loop?

js-test-39

What will be logged to the console?

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Before we analyze the code snippet, let’s try to simplify it by removing the setTimeout.

If we just leave the plain console.log in the loop, then the output is all the values of i from 0 to 4 printed on each iteration of the loop.

However, when the setTimeout is added, the console.log will be executed after the loop has already been processed and the value of i is 5.

As i was declared with var, it has the global scope and the intermediary values aren’t stored in closure around the arrow function () => console.log(i).


ANSWER: the value 5 will be printed on the screen 5 times. Each time on the new line.

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Discussion (3)

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elijahe35939317 profile image
ElijahE

What's the implication on performance?

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coderslang profile image
Coderslang: Become a Software Engineer Author

The question here isn't as much about performance, it's about properly understanding the way asynchronous operations work in JS.

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Coderslang: Become a Software Engineer Author

JavaScript is async by design. There are no immediate problems with it if applied correctly.