Interviewing for a role that requires knowledge of event loop can be tricky. Knowing the right questions to ask about event loop is important for businesses to identify the best candidates for the job. Event loop can be a complex topic to understand, so it is essential to ask the right questions to find the candidate who has a true understanding of the process. This blog post will provide an overview of the key questions to ask when interviewing a candidate with expertise in event loop. From assessing their ability to explain the fundamentals of event loop to gauging their experience with debugging issues related to event loop, these questions can help you find the right hire. Furthermore, this blog post will also provide a few tips to help you make the most of the event loop interview process.
- What is an event loop? …
- Can you explain how a JavaScript event loop works? …
- How do you handle multiple events in the context of an event loop? …
- Can you give me some examples of situations where it’s a good idea to use an event loop? …
- Where are event loops used most often?
JavaScript Interview Question – 3 out of 4 Candidates Fail | Event Loop Concept | by Arnav Gupta
The code is carried out by the event loop, which is also in charge of gathering, processing, and carrying out queued sub-tasks.
As long as there is still code in the application that needs to be run, the Event Loop cycles through the following six phases:
The event loop is named after the way it is typically implemented, which is typically something like:
These six phases create one cycle, or loop, which is known as a tick. A Node.js process exits when there is no more pending work in the Event Loop, or when process.exit() is called manually. A program only runs for as long as there are tasks queued in the Event Loop, or present on the call stack.
As Node. Because JS is an event-driven platform with a single thread, it won’t wait for the laborious task to be finished. It will execute the subsequent task after adding those tasks to the event queue. Finally, if the task is successful, it will receive the callback result.
Understanding a bit how Javascript differs from some other languages.
It’s best to comprehend the outcomes first before trying to comprehend the Javascript event loop. Consider the following examples.
Actions that wait for a response in Javascript, such as setTimeout or AJAX calls, are non-blocking. This implies that while waiting for something to return, the code will continue to run. This is frequently referred to as Javascript’s asynchronous nature. Compare this with a Ruby example below.
The idea to convey is that in Ruby, the sleep action is preventing things from moving forward, even though the code is not 100% equivalent. Similar behavior is experienced with http requests. As a result, until the previous line has finished running, the subsequent lines won’t be evaluated.
Knowing that Javascript is asynchronous allows us to examine the event loop. Essentially, this asynchronous behavior is caused by the event loop. Before continuing, it’s crucial to comprehend a few more Javascript-related concepts.
As a result, each line of code following the previous line of code will be processed by Javascript. We intentionally do not use the word run here. Javascript keeps track of two things to determine when and how to execute lines of code.
Toptal sourced essential questions that the best JavaScript developers and engineers can answer. Driven from our community, we encourage experts to submit questions and offer feedback.
How can the potential pitfall of using typeof bar === “object” to check whether bar is an object be avoided?
The surprising gotcha in JavaScript is that null is also regarded as an object, even though typeof bar === “object” is a trustworthy way to determine whether bar is an object.
Therefore, to the surprise of most developers, the following code will log true (not false) to the console:
As long as this is understood, the issue can be easily avoided by additionally determining whether bar is null:
There are two additional points to mention in order for our response to be comprehensive:
First, if bar is a function, the previous solution will return false. The majority of the time, this is the desired behavior, but if there are times when you also want functions to return true, you could modify the above solution to read as follows:
Second, if bar is an array, the aforementioned solution will return true (e g. , if var bar = [];). Since arrays are in fact objects, this is the desired behavior in the majority of cases. However, if you occasionally want to set the behavior to false for arrays, you can modify the above solution as follows:
However, there is another option that returns true for objects but false for arrays, functions, and nulls:
Or, if you’re using jQuery:
The array case is very straightforward in ES5, which also has a built-in null check:
The code below will output what it does to the console and why.
Most JavaScript developers would anticipate typeof a and typeof b to both be undefined in the example above because both a and b are defined within the function’s enclosing scope and because the line on which they are located starts with the var keyword.
However, that is not the case. The problem here is that most programmers interpret the phrase “var a = b = 3” incorrectly as shorthand for:
However, var a = b = 3; actually stands for:
The output of the code snippet would then be (if strict mode is not being used):
Since the statement var a = b = 3; is a shortcut for the statements b = 3; and var a = b;, b ends up being a global variable (since it is not preceded by the var keyword) and is therefore still in scope even outside of the enclosing function.
Note that, in strict mode (i.e., with use strict), the statement var a = b = 3; will generate a runtime error of ReferenceError: b is not defined, thereby avoiding any headfakes/bugs that might othewise result. (Yet another prime example of why you should use use strict as a matter of course in your code!) 3 .
The code below will output what it does to the console and why.
The above code will output the following to the console:
Since this and self both refer to myObject in the outer function, they can both properly reference and access foo.
However, this is no longer referring to myObject in the inner function. As a result, this. The inner function does not define foo, but the reference to the local variable self is still in scope and accessible there.
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What does it mean and why does a function block contain the entire contents of a JavaScript source file?
A growing number of well-known JavaScript libraries (jQuery, Node) use this technique. js, etc. ). This method creates a closure around the entire contents of the file, creating a private namespace and, perhaps most importantly, preventing name conflicts between various JavaScript modules and libraries.
Another benefit of this approach is that a global variable can have an easy-to-reference (possibly shorter) alias. This is often used, for example, in jQuery plugins. You can use jQuery to remove the $ reference to the jQuery namespace. noConflict(). If this is the case, the following closure technique can still be used in your code to use $:
What does including use strict at the top of a JavaScript source file mean, and what are the advantages?
Use strict is a way to voluntarily enforce stricter parsing and error handling on your JavaScript code at runtime, and that is the short answer and the most significant response. Errors in the code that previously would have been overlooked or would have failed silently now produce errors or throw exceptions. In general, it is a good practice.
Some of the key benefits of strict mode include:
this coercion. Without strict mode, a reference to a this value of null or undefined is automatically coerced to the global. This can cause many headfakes and pull-out-your-hair kind of bugs. In strict mode, referencing a a this value of null or undefined throws an error.function foo(val1, val2, val1){}), thereby catching what is almost certainly a bug in your code that you might otherwise have wasted lots of time tracking down.
- Note: In ECMAScript 5, strict mode used to prevent duplicate property names (e g. var object = (foo: “bar,” foo: “baz,”);) is no longer valid as of ECMAScript 2015, however.
eval() behaves in strict mode and in non-strict mode. Most significantly, in strict mode, variables and functions declared inside of an eval() statement are not created in the containing scope (they are created in the containing scope in non-strict mode, which can also be a common source of problems).delete. The delete operator (used to remove properties from objects) cannot be used on non-configurable properties of the object. Non-strict code will fail silently when an attempt is made to delete a non-configurable property, whereas strict mode will throw an error in such a case.6 .
Consider the two functions below. Why or why not will they both return the same item?
Surprisingly, these two functions will not return the same thing. Rather:
will yield:
This is not only unexpected, but what makes it especially bizarre is that foo2() returns undefined with no error being raised.
This is due to the fact that semicolons in JavaScript are technically optional, despite the fact that doing so is usually very impolite. Consequently, a semicolon is automatically added right after the return statement in foo2() whenever the line containing the return statement (with nothing else on the line) is encountered.
Although the remaining portion of the code is perfectly valid and never executed or invoked (it is merely an unused code block that defines a property bar that is equal to the string “hello”), no error is raised.
This behavior further supports the JavaScript convention that an opening curly brace should be placed at the end of a line rather than at the start of a new one. This becomes more than just a stylistic preference in JavaScript, as this example demonstrates. 7 .
What will the code below output? Explain your answer.
The simple response to this query would be: “You can’t be sure.” it might print out 0. 3 and true, or it might not. JavaScript treats all numbers with floating point precision, which may not always produce the desired outcomes. ”.
The aforementioned case study serves as a classic illustration of this problem. Surprisingly, it will print out:
A common solution is to use the unique constant Number to compare the absolute difference between two numbers. EPSILON:
When the code below is run, the numbers 1-4 will be logged to the console in what order and why?
The values will be logged in the following order:
The parts of this that are presumably more obvious will be explained first:
1 and 4 are displayed first since they are logged by simple calls to console.log() without any delay 2 is displayed after 3 because 2 is being logged after a delay of 1000 msecs (i.e., 1 second) whereas 3 is being logged after a delay of 0 msecs. OK, fine. But shouldn’t 3 be logged before 4, since 4 is being logged by a later line of code, since 3 is being logged after a delay of 0 msecs, doesn’t that mean that it is being logged right away?
The answer has to do with properly understanding JavaScript events and timing.
An event loop in the browser checks the event queue and handles pending events. For example, if an event happens in the background (e. g. while the browser is active (e.g., a script onload event), g. the event is added to the queue after processing an onclick. The queue is checked after the onclick handler is finished, and the event is then handled (e g. , the onload script is executed).
Similar to this, if the browser is busy, setTimeout() also adds the execution of its referenced function to the event queue.
When setTimeout() receives a value of zero as the second argument, it makes an effort to run the specified function “as soon as possible.” Specifically, the function’s execution is added to the event queue for the following timer tick. However, be aware that this does not happen instantly; the function is not used until the subsequent tick. That’s why in the above example, the call to console. log(4) occurs before the call to console. log(3) (since the call to console. log(3) is invoked via setTimeout, so it is slightly delayed). 9 .
Write a simple function (less than 160 characters) that returns a boolean indicating whether or not a string is a palindrome.
If str is a palindrome, the following one-line function will return true; otherwise, it will return false.
For example:
Create a sum method that will function as intended when called using either of the following syntaxes.
There are (at least) two ways to do this:
METHOD 1
The actual arguments passed to a function are accessible through an arguments object that functions in JavaScript provide access to. As a result, we can use the length property to determine the function’s argument count in real time.
In the event that two arguments are passed, we simply add them and return.
If not, we assume it was called with the syntax sum(2)(3), in which case we return an anonymous function that adds the arguments passed to both sum() and the anonymous function (in this case, arguments 2 and 3).
METHOD 2
JavaScript does not require the number of arguments to match the number in the function definition when a function is called. The extra arguments will just be ignored if the number passed is greater than the number of arguments specified in the function definition. The missing arguments, however, will have a value of undefined when referenced within the function if the number of arguments passed is less than the number of arguments in the function definition. We can therefore determine how the function was called in the example above by simply examining whether the second argument is undefined and acting accordingly. 11 .
Consider the following code snippet:
(a) What happens when the user clicks “Button 4”, and why, is it recorded in the console?
(b) Offer one or more different implementations that will function as anticipated.
(a) The number 5 will always be logged to the console regardless of which button the user clicks. This is due to the fact that the for loop has already ended when the onclick method is called (for any of the buttons), and the variable i already has a value of 5. (The interviewee receives bonus points if they can discuss how execution contexts, variable objects, activation objects, and the internal “scope” property affect the closure behavior. ).
(b) The key to making this work is to pass the value of i into a newly created function object each time the for loop is executed. Here are four possible ways to accomplish this:
Alternatively, you could wrap the entire call to btn. addEventListener in the new anonymous function:
Alternatively, we could use the native forEach method of the array object in place of the for loop:
Finally, using let i rather than var i in an ES6/ES2015 context is the most straightforward fix:
Assuming d is an “empty” object in scope, say:
…what is accomplished using the following code?
Two properties are set on the object d by the snippet of code above. The ideal outcome of any lookup on a JavaScript object with an unset key is undefined. However, running this code declares those properties as the object’s “own properties.”
This is a helpful tactic for guaranteeing that an object possesses a particular set of properties. Passing this object to Object. even if their values are undefined, keys will return an array that includes the set keys. 13 .
The code below will output what it does to the console and why.
The logged output will be:
arr1 and arr2 are the same (i. e. [n,h,o,j, [j,o,n,e,s]]) following the execution of the aforementioned code for the following reasons:
reverse() method doesn’t only return the array in reverse order, it also reverses the order of the array itself (i.e., in this case, arr1). reverse() method returns a reference to the array itself (i.e., in this case, arr1). As a result, arr2 is simply a reference to (rather than a copy of) arr1. Therefore, when anything is done to arr2 (i.e., when we invoke arr2.push(arr3);), arr1 will be affected as well since arr1 and arr2 are simply references to the same object. And here are a couple of unrelated points that occasionally cause people trouble when responding to this query:
push() method of another array pushes that entire array as a single element onto the end of the array. As a result, the statement arr2.push(arr3); adds arr3 in its entirety as a single element to the end of arr2 (i.e., it does not concatenate the two arrays, that’s what the concat() method is for). slice() as a way of referencing elements at the end of the array; e.g., a subscript of -1 indicates the last element in the array, and so on. 14 .
The code below will output what it does to the console, and why.
The above code will output the following to the console:
The main problem here is that JavaScript (ECMAScript) automatically types values to fit the operation being done because it is a loosely typed language. Let’s examine how each of the aforementioned examples relates to this.
Example 1: The result of 1 + “2” + “2” is “122.” Explanation: The first operation in 1 + “2” JavaScript assumes it needs to perform string concatenation because one of the operands (“2”) is a string and changes the type of 1 to “1,” so 1 + “2” produces “12”. Then, “12” + “2” yields “122”.
Explanation: According to the order of operations, the first operation to be carried out is +”2″ (the additional + before the first “2” is treated as a unary operator). JavaScript does this by changing the type of “2” to numeric and adding the unary + sign to it (i e. , treats it as a positive number). The subsequent operation is therefore 1 + 2, which naturally results in 3. Then again, there is an operation between a string and a number (i e. , 3 and “2”), so once more JavaScript performs string concatenation and converts the type of the numeric value to a string, resulting in “32”.
Example 3: 1 + -“1” + “2” Explanation: This example’s explanation is the same as the first one, with the exception that the unary operator is – rather than +. The result is “02.” In other words, “1” becomes 1, which, after the – operation, turns into “-1,” which is added to “1” to produce “0,” which is then made into a string and concatenated with the last “2” operand to produce “02.”
Explanation for Example 4: +”1″ + “1” + “2” Outputs: “112” The first “1” operand is typecast to a numeric value based on the unary + operator that comes before it, but it is immediately converted back to a string when it is concatenated with the second “1” operand, which is then concatenated with the final “2” operand, producing the string “112”.
Explanation: Since the – operator cannot be used on strings and neither “A” nor “B” can be converted to numeric values, “A” – “B” yields NaN, which is then concatenated with the string “2” to produce “NaN2.”
Example 6: “A” – “B” + 2 Results in NaN Just as in Example 5, “A” – “B” results in NaN. However, if any operation is performed on NaN with any other numeric operand, NaN will still result. 15 .
In the event that the array list is too big, the recursive code below will result in a stack overflow. How can this be fixed while preserving the recursive pattern?
The nextListItem function can be changed as shown below to prevent the potential stack overflow:
Because the event loop, rather than the call stack, manages the recursion, the stack overflow is prevented. The timeout function (nextListItem) is pushed to the event queue and the function exits when nextListItem runs, clearing the call stack if item is not null. The next item is processed after the event queue runs its timed-out event, and a timer is then set to call nextListItem once more. Because there is no direct recursive call made during the processing of the method, the call stack is kept clear regardless of the number of iterations. 16 .
What is a “closure” in JavaScript? Provide an example.
An inner function known as a closure has access to the variables in the scope chain of the outer (enclosing) function. Three different scopes of variables are accessible to the closure: its own scope, the scope of the enclosing function, and global variables.
Here is an example:
The global namespace, innerFunc, and outerFunc variables are all in scope in the innerFunc in the example above. The above code will therefore produce the following output:
The following lines of code would output something to the console.
The code will output the following four lines:
Both || and && are logical operators in JavaScript that, when evaluated from left to right, return the first fully-determined “logical value.”
The or (||) operator. X is first evaluated and treated as a boolean value in an expression of the form X||Y. As the “or” condition has already been satisfied if this boolean value is true, true (1) is returned and Y is not evaluated. However, if this boolean value is “false,” we won’t understand whether X||Y is true or false until we evaluate Y and interpret it as a boolean value as well.
As a result, both 0 || 1 and 1 || 2 evaluate to true (1).
The and (&&) operator. The first step in evaluating and interpreting X as a boolean value is done in an expression of the form X&&Y. As the “and” condition has already failed, if this boolean value is false, false (0) is returned and Y is not evaluated. However, even if this boolean value is “true,” we won’t know whether X&&Y is true or false until we evaluate Y and interpret it as a boolean value as well.
But the intriguing feature of the && operator is that it returns the expression itself when it is evaluated as “true.” This is acceptable because it counts as “true” in logical expressions and can also be utilized to return that value if desired. This explains why, somewhat unexpectedly, 1 && 2 returns 2 (instead of returning true or 1). 18 .
What result will the following code produce when it is run?
The code will output:
In JavaScript, there are two sets of equality operators. When two expressions on either side of the triple-equal operator === have the same type and value, it evaluates to true, just like any other traditional equality operator. However, the double-equal operator attempts to force the values prior to comparison. Therefore, it is generally recommended to use the === instead of the == The same holds true for !== vs !=. 19 .
Explain your response to the following code’s output.
The output of this code will be 456 (not 123).
The following explains why: When setting a parameter value for an object property, JavaScript will automatically stringify the value. Since both b and c are objects in this situation, they will both be changed to “[object Object]”. As a result, a[b] and a[c] are both interchangeable and equivalent to a[“[object Object]] In light of this, setting or referencing a[c] is identical to doing the same for a[b]. 20 .
What will the following code output to the console:
The code will output the value of 10 factorial (i. e. , 10!, or 3,628,800).
Here’s why:
Recursively, the named function f() calls itself until it calls f(1), which just returns 1. Here, therefore, is what this does:
Consider the code snippet below. What will the console output be and why?.
Despite the fact that x is never set in the inner function, the output will always be 1. Here’s why:
A closure is a function that includes all variables or functions that were in-scope when the closure was created, as described in our JavaScript Hiring Guide. A closure is implemented as an “inner function” in JavaScript; i e. , a function defined within the body of another function. The fact that an inner function can still access the variables of the outer function is a crucial aspect of closures.
Since x in this example is not defined in the inner function, it is sought out in the outer function’s scope, where it is discovered to have a value of 1. 22 .
What will be displayed on the console by the following code, and why:
What’s wrong with this code, and how do we fix it?
The code will output:
The first console. Because we are extracting the method from the hero object, stoleSecretIdentity() is being called in the global context, which results in log printing undefined (i e. where the _name property is absent (for example, the window object
One way to fix the stoleSecretIdentity() function is as follows:
Make a function that visits all of the descendents of a given DOM Element on the page, not just its immediate children. The function should pass each element it visits to a callback function that is provided.
The arguments to the function should be:
Visiting all elements in a tree (DOM) is a classic Depth-First-Search algorithm application. Here’s an example solution:
What does the following JavaScript code produce as a test of your knowledge?
Output:
Why isn’t it 10 and 5?
First of all, because fn is a parameter passed to the function method, its scope (this) is window. var length = 10; is declared at the window level. It also can be accessed as window. length or length or this. length (when this === window. ).
method is bound to Object obj, and obj. method is called with parameters fn and 1. Although a method only accepts one parameter, when it is invoked, two additional parameters are passed, one of which is a function callback and the other is simply a number.
When fn() is called inside of a method that received the function as a global parameter, this Rather than the length of 5 as specified in Object obj, length will have access to the var length = 10 (declared globally).
Now that we know, we can use the arguments[] array to access any number of arguments in a JavaScript function.
Hence arguments[0]() is nothing but calling fn(). The arguments array is now the scope of this function inside fn, and logging the length of arguments[] will return 2.
Hence the output will be as above. 25 .
Consider the following code. What will the output be, and why?.
Even when contained within a with or catch block, var statements are raised to the top of the global or function scope they belong to (without their value initialization). The error’s identifier, however, is only discernible within the catch block. It is equivalent to:
What will be the output of this code?
Neither 21, nor 20, the result is undefined
It’s because JavaScript initialization is not hoisted.
When the function is executed, it checks to see if a local x variable is present but hasn’t yet been declared, so it won’t look for a global one and displays the local value of 21. ) 27 .
★ questions ★ a wizard zines project
Hello! Here are some questions & answers. The goal isnt to get all the questions “right”. If you find a subject that interests you, I’d advise you to look it up and learn more about it because the point is to learn something.
FAQ
How do you explain an event loop?
When the function stack is empty, an event loop is something that removes items from the queue and puts them on the function execution stack.
What is event loop iteration?
The Call Stack and the Callback Queue are two straightforward tasks that the Event Loop performs. The Event Loop will pull the first event from the queue and push it to the Call Stack if it is empty, which effectively runs it. In the Event Loop, such an iteration is referred to as a tick.
What is the use case of event loop?
This is made easier by the event loop, which continuously determines whether the call stack is empty. If it is empty, the event queue is used to add new functions. The current function call is handled if it is not.
What is an event loop asynchronous?
The event loop is the core of every asyncio application. Event loops carry out network IO operations, execute subprocesses, and run asynchronous tasks and callbacks. The high-level asyncio functions, such as asyncio, should typically be used by application developers.
