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What will output these example? Why?

for (var i=0; i < 10; i++){
    }, 1000);

How to fix it to output numbers from 0 to 9?


Using let you will get output numbers from 0 to 9.


Feel free to run it and prove me wrong, but I believe that it prints 10 ten times, at 1 second intervals.

It's due to binding on the variable I, not the value of I when you create the lambda.

You need to introduce a local variable in the loop body to fix it.

You are wrong, if you use for(let i instead of for(var i it will print 0 to 9 correctly.

Can you explain why that happens? I'm new to ES6 and learned that let has block scope. When used in the for loop, in conjunction with setTimeOut set to 1000, you would think the console.log would run every 1000 ms for each console.log. But this doesn't happen. When I tried it, the sequence of console.logs appear all at once after 1000 ms. Why does that happen? Is it because setTimeOut, when called 10 times, gets pushed to the stack, then after 1000 ms the stack does its thing and all 10 calls are executed simultaneously?

If you take out the setTimeOut and just do console.log within the for (var i loop you get the correct result as well.

Yup, you're correct. The callbacks are registered very close to each other - they're all queued up to run at basically the same time.

  • Using let

for(let i=0; i<10; i++) {

  • Using bind

for(var i=0; i<10;i++) {
setTimeout(console.log.bind(null,i), i*100);

  • Or you can use IIFE

for(var i=0; i<10; i++) {
setTimeout(()=> console.log(x), x*100)


On the bind example, heads up that in some older browsers you need to pass console as the first argument to any console.<method>.bind(). Not that this is a particularly real-world thing to do, but it's bitten me a few times when trying to debug a promise chain in an older browser; I always love to do promise.then(console.log), but in old browsers this breaks until you do promise.then(console.log.bind(console)).


It is an excellent question to test how much you understand JS.


for (var i=0; i<10; i++) {
setTimeout(function() { console.log(i++ - 10); }, 1000);


Fun trivia: setTimeout can have more than two arguments. After the callback function, and the duration, you can pass args for the callback, as well. Instead of working around setTimeout, use setTimeout.

for (var i = 0; i < 10; i++) {
   setTimeout(console.log, 100*i, i); 

(Although I don't even write the for loop, like, ever. It's all map, filter, reduce etc. So I'd rewrite it into something like this:

new Array(10)
   .map((x, i) => i)
   .forEach(x => setTimeout(console.log, 100*x, x))

Because answer on this question will discover what answerer know about:

  • Scopes
  • Closures
  • How event loop works in the browser

This question is perfect to sieve juniors from those who never ever coded.


For candidates with a clear focus on a specific technology/library/framework/… I always ask "In which case would you not use that?".

Or a similar version of that question.

Like "For which kind of project would Angular be a bad architectural choice?".

Because one size never fits all and it's always a good discussion-starter.

I don't ask weird syntax-questions. It's just rude. It doesn't tell me anything at all about the person. It's exploiting their nervousness for no gain. I want to know if they will make the right calls, and produce working, readable and tested solutions. You know, the stuff that is not one google-search away.


Solid approach.

Remember, interviewing is a two-way street. It's just as much about you assessing a prospective employer/boss as it is them assessing you.

If an interviewer tries to "catch you out" with some arbitrary, illegible code or tries to belittle you because you don't know answers to some esoteric problem then it says more about them and their approaches to work than it does about your knowledge/abilities.


Most def, it's more a conversation-starter, without a "right" answer. If they can argue their claim, perfect.

I think I could argue why React should always be used, sometimes be used, and never be used :-)

Absolutely. Evidence about ways of thinking/approaching a problem are much more revealing about a candidate than their ability to interpret obtuse code like a compiler.


100% agreed on this. All the "trick questions" in this thread are basically intended for the interviewer to flex on the candidate.

A better twist on these would be "You write this code, and instead of getting X behavior like you wanted, you get Y behavior. How would you go about debugging this?"

Plus, the questions about "When do you not use this" are also great, because not only do you weed out candidates that are fully invested in one popular thing, but you also get some information on their priorities in choosing a tool for a job.


What will be output of

(() => console.log(this)).bind("done")()

var fx = (() => console.log(this));"done");

Try in chrome, you will not see "done"

(() => console.log(arguments))("a");

Great question! I believe this is because arrow functions can't be bound and also don't have arguments, for that we need to use regular function definitions:

(function() { console.log(this) }).bind("done")();

var fx = (function() { console.log(this) });"done");

(function() { console.log(arguments) })("a");

You can't bind anything to an arrow functions this. But you can bind arguments to them. ie:

((arg) => console.log(arg)).bind(null, "done")()

would work.


I always like to ask which project the candidate is most proud of, like what's their "darling" and why is that so. This always opens room to see what they care for, how can they articulate that and opens room for more questions.


Yay, that's also my favourite interview question... and I hope to be asked that question, too, when I'm the candidate.


I'd ask what the candidate used for build tooling in personal projects and any previous team projects. I think the answer to this question would tell me a lot about how they approached their work and would lead to an insightful conversation.


I very rarely do JS interviews, but when I do I usually ask to implement somewhat from modern libraries in pure JS. Just one feature.

For juniors, it’d be kinda _.tail from lodash.
For middles, somewhat $(".class") from jQuery.
For seniors, nowadays it’s a web component (optionally with bindings.)


I know this might sound a bit ridiculous, but...

'When was the first time you touched a computer?'

'And when was the first time a computer touched you?'

No pun intended! :)


What's difference between this

kung.prototype.fu = function() { juli.do_the_thing(); 

and this

class kung {

  fu() { 

What's your favorite addition to ECMAscript?


Create a non-brute force autocomplete function in JavaScript. 😎


Implement Promises using function generators only.


Beside your question, Can i ask where it becomes usefull to use generators? i know about them but haven't used yet in production code.

-Performance optimizations maybe?


They are useful wherever you want to suspend and handle control (and optionally a value) back to the caller. When the caller is done doing its stuff, he can decide to give you back control (also optionally with a value). Your function continues where it has stopped before.

A prominent use case are CSP (communicating sequencial processes) style concurrency models, where multiple processes read and write to and from a shared channel. Processes can block and wait until some specified message type is observed on the channel. On the other hand, processes can write to the channel, without caring if there is another process that would make use of the message (you could write error log messages to the channel but only display them in dev mode and otherwise send them to a remote endpoint for further processing them in e.g. Kibana).

The great advantage of this model is the loose coupling that allows you to simply swap out or replace parts of your application depending on your needs.

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