ValueLabs JavaScript Most Frequently Asked Latest Interview Questions Answers
What will the code below output? Explain your answer.
console.log(0.1 + 0.2);
console.log(0.1 + 0.2 == 0.3);
An educated answer to this question would simply be: “You can’t be sure. it might print out 0.3 and true, or it might not. Numbers in JavaScript are all treated with floating point precision, and as such, may not always yield the expected results.”
The example provided above is classic case that demonstrates this issue. Surprisingly, it will print out:
0.30000000000000004
false
A typical solution is to compare the absolute difference between two numbers with the special constant Number.EPSILON:
function areTheNumbersAlmostEqual(num1, num2) {
return Math.abs( num1 - num2 ) < Number.EPSILON;
}
console.log(areTheNumbersAlmostEqual(0.1 + 0.2, 0.3));
Discuss possible ways to write a function isInteger(x) that determines if x is an integer.
This may sound trivial and, in fact, it is trivial with ECMAscript 6 which introduces a new Number.isInteger() function for precisely this purpose. However, prior to ECMAScript 6, this is a bit more complicated, since no equivalent of the Number.isInteger() method is provided.
The issue is that, in the ECMAScript specification, integers only exist conceptually; i.e., numeric values are always stored as floating point values.
With that in mind, the simplest and cleanest pre-ECMAScript-6 solution (which is also sufficiently robust to return false even if a non-numeric value such as a string or null is passed to the function) would be the following use of the bitwise XOR operator:
function isInteger(x) { return (x ^ 0) === x; }
The following solution would also work, although not as elegant as the one above:
function isInteger(x) { return Math.round(x) === x; }
Note that Math.ceil() or Math.floor() could be used equally well (instead of Math.round()) in the above implementation.
Or alternatively:
function isInteger(x) { return (typeof x === 'number') && (x % 1 === 0); }
One fairly common incorrect solution is the following:
function isInteger(x) { return parseInt(x, 10) === x; }
While this parseInt-based approach will work well for many values of x, once x becomes quite large, it will fail to work properly. The problem is that parseInt() coerces its first parameter to a string before parsing digits. Therefore, once the number becomes sufficiently large, its string representation will be presented in exponential form (e.g., 1e+21). Accordingly, parseInt() will then try to parse 1e+21, but will stop parsing when it reaches the e character and will therefore return a value of 1. Observe:
> String(1000000000000000000000)
'1e+21'
> parseInt(1000000000000000000000, 10)
1
> parseInt(1000000000000000000000, 10) === 1000000000000000000000
false
In what order will the numbers 1-4 be logged to the console when the code below is executed? Why?
(function() {
console.log(1);
setTimeout(function(){console.log(2)}, 1000);
setTimeout(function(){console.log(3)}, 0);
console.log(4);
})();
The values will be logged in the following order:
1
4
3
2
Let’s first explain the parts of this that are presumably more obvious:
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 if 3 is being logged after a delay of 0 msecs, doesn’t that mean that it is being logged right away? And, if so, shouldn’t it be logged before 4, since 4 is being logged by a later line of code?
The answer has to do with properly understanding JavaScript events and timing.
The browser has an event loop which checks the event queue and processes pending events. For example, if an event happens in the background (e.g., a script onload event) while the browser is busy (e.g., processing an onclick), the event gets appended to the queue. When the onclick handler is complete, the queue is checked and the event is then handled (e.g., the onload script is executed).
Similarly, setTimeout() also puts execution of its referenced function into the event queue if the browser is busy.
When a value of zero is passed as the second argument to setTimeout(), it attempts to execute the specified function “as soon as possible”. Specifically, execution of the function is placed on the event queue to occur on the next timer tick. Note, though, that this is not immediate; the function is not executed until the next 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).
Write a simple function (less than 160 characters) that returns a boolean indicating whether or not a string is a palindrome.
The following one line function will return true if str is a palindrome; otherwise, it returns false.
function isPalindrome(str) {
str = str.replace(/\W/g, '').toLowerCase();
return (str == str.split('').reverse().join(''));
}
For example:
console.log(isPalindrome("level")); // logs 'true'
console.log(isPalindrome("levels")); // logs 'false'
console.log(isPalindrome("A car, a man, a maraca")); // logs 'true'
Write a sum method which will work properly when invoked using either syntax below.
console.log(sum(2,3)); // Outputs 5
console.log(sum(2)(3)); // Outputs 5
There are (at least) two ways to do this:
METHOD 1
function sum(x) {
if (arguments.length == 2) {
return arguments[0] + arguments[1];
} else {
return function(y) { return x + y; };
}
}
In JavaScript, functions provide access to an arguments object which provides access to the actual arguments passed to a function. This enables us to use the length property to determine at runtime the number of arguments passed to the function.
If two arguments are passed, we simply add them together and return.
Otherwise, we assume it was called in the form sum(2)(3), so we return an anonymous function that adds together the argument passed to sum() (in this case 2) and the argument passed to the anonymous function (in this case 3).
METHOD 2
function sum(x, y) {
if (y !== undefined) {
return x + y;
} else {
return function(y) { return x + y; };
}
}
When a function is invoked, JavaScript does not require the number of arguments to match the number of arguments in the function definition. If the number of arguments passed exceeds the number of arguments in the function definition, the excess arguments will simply be ignored. On the other hand, if the number of arguments passed is less than the number of arguments in the function definition, the missing arguments will have a value of undefined when referenced within the function. So, in the above example, by simply checking if the 2nd argument is undefined, we can determine which way the function was invoked and proceed accordingly.
Consider the following code snippet:
for (var i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', function(){ console.log(i); });
document.body.appendChild(btn);
}
(a) What gets logged to the console when the user clicks on “Button 4” and why?
(b) Provide one or more alternate implementations that will work as expected.
(a) No matter what button the user clicks the number 5 will always be logged to the console. This is because, at the point that the onclick method is invoked (for any of the buttons), the for loop has already completed and the variable i already has a value of 5. (Bonus points for the interviewee if they know enough to talk about how execution contexts, variable objects, activation objects, and the internal “scope” property contribute to the closure behavior.)
(b) The key to making this work is to capture the value of i at each pass through the for loop by passing it into a newly created function object. Here are four possible ways to accomplish this:
for (var i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', (function(i) {
return function() { console.log(i); };
})(i));
document.body.appendChild(btn);
}
Alternatively, you could wrap the entire call to btn.addEventListener in the new anonymous function:
for (var i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
(function (i) {
btn.addEventListener('click', function() { console.log(i); });
})(i);
document.body.appendChild(btn);
}
Or, we could replace the for loop with a call to the array object’s native forEach method:
['a', 'b', 'c', 'd', 'e'].forEach(function (value, i) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', function() { console.log(i); });
document.body.appendChild(btn);
});
Lastly, the simplest solution, if you’re in an ES6/ES2015 context, is to use let i instead of var i:
for (let i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', function(){ console.log(i); });
document.body.appendChild(btn);
}
What are the advantages of Ajax?
Following are the advantages of Ajax:
Bandwidth utilization – It saves memory when the data is fetched from the same page.
More interactive
Speeder retrieval of data
What is JSON in Ajax?
JSON is abbreviated as JavaScript Object Notation.
JSON is a safe and reliable data interchange format in JavaScript, which is easy to understand for both users and machines.
What is AJAX Control Extender Toolkit?
AJAX Control Toolkit is one of the extenders that are used to extend or add the functionalities of the ASP.NET controls. The extenders use a block of JavaScript code to add new and enhanced capabilities to the ASP.NET controls.
AJAX Control Extender Toolkit is a free download from site.
Where AJAX cannot be used?
Users cannot use AJAX if
If Page need to show in a search engine
If browser does not support JavaScript
If user wants to create secure application
What are the difference between AJAX and Javascript?
The differences between AJAX and JavaScript are as follows:
AJAX
Javascript
AJAX sends request to the server and does not wait for the response. It performs other operations on the page during that time JavaScript make a request to the server and waits for response
AJAX does not require the page to refresh for downloading the whole page JavaScript manages and controls a Web page after being downloaded
AJAX minimizes the overload on the server since the script needs to request once JavaScript posts a request that updates the script every time
What are the components of the ASP.NET AJAX architecture?
There are two components of AJAX Architecture:
AJAX client architecture
AJAX server architecture
What are the extender controls?
The extender controls uses a block of JavaScript code to add new and enhanced capabilities to ASP.NET.
What are the pre-requisites to execute AJAX applications on a server?
AJAX is a built-in functionality of .NET Framework 4.0 and AJAX application can be executed by just installing Microsoft Visual Studio 2010. To use extenders in your applications, you are required to install AJAX Control Toolkit and copy the AjaxControlToolkit.dll file to the Bin directory of your application.
Which request is better, Get or Post?
AJAX requests should use an HTTP GET request where the data does not change for a given URL requested.
An HTTP POST should be used when state is updated on the server. This is highly recommended for a consistent web application architecture.
What are the limitations of Ajax?
An Ajax Web Application tends to confuse end users if the network bandwidth is slow and there is no full postback running.
What is the use of the ‘this’ keyword?
The keyword ‘this’ refers to the current instance of the object when used inside a function. But, when used outside a function, it refers to the window object.
Is Exception handling possible in Javascript?
With the latest version of Javascript, exception handling is possible; and this can be achieved using the following keywords try, catch and finally.
How is it possible to get the total number of arguments that are passed to a function?
The arguments.length property helps in getting the total number of arguments that are passed to a function.
What is the difference between typeof and instanceof operators in Javascript?
The typeof operator returns a string of what type the operand is. Whereas, the instanceof operator does not work with primitive data types; but works with objects and checks on what type the object is.
What is DOM?
DOM (Document Object Model) is an object-oriented representation of the HTML elements. All the elements (or nodes) are part of window.document.
What will the code below output? Explain your answer.
console.log(0.1 + 0.2);
console.log(0.1 + 0.2 == 0.3);
An educated answer to this question would simply be: “You can’t be sure. it might print out 0.3 and true, or it might not. Numbers in JavaScript are all treated with floating point precision, and as such, may not always yield the expected results.”
The example provided above is classic case that demonstrates this issue. Surprisingly, it will print out:
0.30000000000000004
false
A typical solution is to compare the absolute difference between two numbers with the special constant Number.EPSILON:
function areTheNumbersAlmostEqual(num1, num2) {
return Math.abs( num1 - num2 ) < Number.EPSILON;
}
console.log(areTheNumbersAlmostEqual(0.1 + 0.2, 0.3));
 |
| ValueLabs JavaScript Most Frequently Asked Latest Interview Questions Answers |
Discuss possible ways to write a function isInteger(x) that determines if x is an integer.
This may sound trivial and, in fact, it is trivial with ECMAscript 6 which introduces a new Number.isInteger() function for precisely this purpose. However, prior to ECMAScript 6, this is a bit more complicated, since no equivalent of the Number.isInteger() method is provided.
The issue is that, in the ECMAScript specification, integers only exist conceptually; i.e., numeric values are always stored as floating point values.
With that in mind, the simplest and cleanest pre-ECMAScript-6 solution (which is also sufficiently robust to return false even if a non-numeric value such as a string or null is passed to the function) would be the following use of the bitwise XOR operator:
function isInteger(x) { return (x ^ 0) === x; }
The following solution would also work, although not as elegant as the one above:
function isInteger(x) { return Math.round(x) === x; }
Note that Math.ceil() or Math.floor() could be used equally well (instead of Math.round()) in the above implementation.
Or alternatively:
function isInteger(x) { return (typeof x === 'number') && (x % 1 === 0); }
One fairly common incorrect solution is the following:
function isInteger(x) { return parseInt(x, 10) === x; }
While this parseInt-based approach will work well for many values of x, once x becomes quite large, it will fail to work properly. The problem is that parseInt() coerces its first parameter to a string before parsing digits. Therefore, once the number becomes sufficiently large, its string representation will be presented in exponential form (e.g., 1e+21). Accordingly, parseInt() will then try to parse 1e+21, but will stop parsing when it reaches the e character and will therefore return a value of 1. Observe:
> String(1000000000000000000000)
'1e+21'
> parseInt(1000000000000000000000, 10)
1
> parseInt(1000000000000000000000, 10) === 1000000000000000000000
false
In what order will the numbers 1-4 be logged to the console when the code below is executed? Why?
(function() {
console.log(1);
setTimeout(function(){console.log(2)}, 1000);
setTimeout(function(){console.log(3)}, 0);
console.log(4);
})();
The values will be logged in the following order:
1
4
3
2
Let’s first explain the parts of this that are presumably more obvious:
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 if 3 is being logged after a delay of 0 msecs, doesn’t that mean that it is being logged right away? And, if so, shouldn’t it be logged before 4, since 4 is being logged by a later line of code?
The answer has to do with properly understanding JavaScript events and timing.
The browser has an event loop which checks the event queue and processes pending events. For example, if an event happens in the background (e.g., a script onload event) while the browser is busy (e.g., processing an onclick), the event gets appended to the queue. When the onclick handler is complete, the queue is checked and the event is then handled (e.g., the onload script is executed).
Similarly, setTimeout() also puts execution of its referenced function into the event queue if the browser is busy.
When a value of zero is passed as the second argument to setTimeout(), it attempts to execute the specified function “as soon as possible”. Specifically, execution of the function is placed on the event queue to occur on the next timer tick. Note, though, that this is not immediate; the function is not executed until the next 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).
Write a simple function (less than 160 characters) that returns a boolean indicating whether or not a string is a palindrome.
The following one line function will return true if str is a palindrome; otherwise, it returns false.
function isPalindrome(str) {
str = str.replace(/\W/g, '').toLowerCase();
return (str == str.split('').reverse().join(''));
}
For example:
console.log(isPalindrome("level")); // logs 'true'
console.log(isPalindrome("levels")); // logs 'false'
console.log(isPalindrome("A car, a man, a maraca")); // logs 'true'
Write a sum method which will work properly when invoked using either syntax below.
console.log(sum(2,3)); // Outputs 5
console.log(sum(2)(3)); // Outputs 5
There are (at least) two ways to do this:
METHOD 1
function sum(x) {
if (arguments.length == 2) {
return arguments[0] + arguments[1];
} else {
return function(y) { return x + y; };
}
}
In JavaScript, functions provide access to an arguments object which provides access to the actual arguments passed to a function. This enables us to use the length property to determine at runtime the number of arguments passed to the function.
If two arguments are passed, we simply add them together and return.
Otherwise, we assume it was called in the form sum(2)(3), so we return an anonymous function that adds together the argument passed to sum() (in this case 2) and the argument passed to the anonymous function (in this case 3).
METHOD 2
function sum(x, y) {
if (y !== undefined) {
return x + y;
} else {
return function(y) { return x + y; };
}
}
When a function is invoked, JavaScript does not require the number of arguments to match the number of arguments in the function definition. If the number of arguments passed exceeds the number of arguments in the function definition, the excess arguments will simply be ignored. On the other hand, if the number of arguments passed is less than the number of arguments in the function definition, the missing arguments will have a value of undefined when referenced within the function. So, in the above example, by simply checking if the 2nd argument is undefined, we can determine which way the function was invoked and proceed accordingly.
Consider the following code snippet:
for (var i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', function(){ console.log(i); });
document.body.appendChild(btn);
}
(a) What gets logged to the console when the user clicks on “Button 4” and why?
(b) Provide one or more alternate implementations that will work as expected.
(a) No matter what button the user clicks the number 5 will always be logged to the console. This is because, at the point that the onclick method is invoked (for any of the buttons), the for loop has already completed and the variable i already has a value of 5. (Bonus points for the interviewee if they know enough to talk about how execution contexts, variable objects, activation objects, and the internal “scope” property contribute to the closure behavior.)
(b) The key to making this work is to capture the value of i at each pass through the for loop by passing it into a newly created function object. Here are four possible ways to accomplish this:
for (var i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', (function(i) {
return function() { console.log(i); };
})(i));
document.body.appendChild(btn);
}
Alternatively, you could wrap the entire call to btn.addEventListener in the new anonymous function:
for (var i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
(function (i) {
btn.addEventListener('click', function() { console.log(i); });
})(i);
document.body.appendChild(btn);
}
Or, we could replace the for loop with a call to the array object’s native forEach method:
['a', 'b', 'c', 'd', 'e'].forEach(function (value, i) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', function() { console.log(i); });
document.body.appendChild(btn);
});
Lastly, the simplest solution, if you’re in an ES6/ES2015 context, is to use let i instead of var i:
for (let i = 0; i < 5; i++) {
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Button ' + i));
btn.addEventListener('click', function(){ console.log(i); });
document.body.appendChild(btn);
}
What are the advantages of Ajax?
Following are the advantages of Ajax:
Bandwidth utilization – It saves memory when the data is fetched from the same page.
More interactive
Speeder retrieval of data
What is JSON in Ajax?
JSON is abbreviated as JavaScript Object Notation.
JSON is a safe and reliable data interchange format in JavaScript, which is easy to understand for both users and machines.
What is AJAX Control Extender Toolkit?
AJAX Control Toolkit is one of the extenders that are used to extend or add the functionalities of the ASP.NET controls. The extenders use a block of JavaScript code to add new and enhanced capabilities to the ASP.NET controls.
AJAX Control Extender Toolkit is a free download from site.
Where AJAX cannot be used?
Users cannot use AJAX if
If Page need to show in a search engine
If browser does not support JavaScript
If user wants to create secure application
What are the difference between AJAX and Javascript?
The differences between AJAX and JavaScript are as follows:
AJAX
Javascript
AJAX sends request to the server and does not wait for the response. It performs other operations on the page during that time JavaScript make a request to the server and waits for response
AJAX does not require the page to refresh for downloading the whole page JavaScript manages and controls a Web page after being downloaded
AJAX minimizes the overload on the server since the script needs to request once JavaScript posts a request that updates the script every time
What are the components of the ASP.NET AJAX architecture?
There are two components of AJAX Architecture:
AJAX client architecture
AJAX server architecture
What are the extender controls?
The extender controls uses a block of JavaScript code to add new and enhanced capabilities to ASP.NET.
What are the pre-requisites to execute AJAX applications on a server?
AJAX is a built-in functionality of .NET Framework 4.0 and AJAX application can be executed by just installing Microsoft Visual Studio 2010. To use extenders in your applications, you are required to install AJAX Control Toolkit and copy the AjaxControlToolkit.dll file to the Bin directory of your application.
Which request is better, Get or Post?
AJAX requests should use an HTTP GET request where the data does not change for a given URL requested.
An HTTP POST should be used when state is updated on the server. This is highly recommended for a consistent web application architecture.
What are the limitations of Ajax?
An Ajax Web Application tends to confuse end users if the network bandwidth is slow and there is no full postback running.
What is the use of the ‘this’ keyword?
The keyword ‘this’ refers to the current instance of the object when used inside a function. But, when used outside a function, it refers to the window object.
Is Exception handling possible in Javascript?
With the latest version of Javascript, exception handling is possible; and this can be achieved using the following keywords try, catch and finally.
How is it possible to get the total number of arguments that are passed to a function?
The arguments.length property helps in getting the total number of arguments that are passed to a function.
What is the difference between typeof and instanceof operators in Javascript?
The typeof operator returns a string of what type the operand is. Whereas, the instanceof operator does not work with primitive data types; but works with objects and checks on what type the object is.
What is DOM?
DOM (Document Object Model) is an object-oriented representation of the HTML elements. All the elements (or nodes) are part of window.document.
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