Name some of the Javascript frameworks.
There are many Javascript Frameworks available today, but the most commonly used frameworks are:
(i) Angular (ii) React (iii) Vue
Explain the typeof operator.
The operator typeof is an example of Unary Operators, which is used by placing it before its operand; which can be of any type.
What are anonymous functions in Javascript?
An anonymous function allows a developer to create a function that has no name. In other words, anonymous functions can be used to store a bit of functionality in a variable and pass that piece of functionality around.
What is the difference between an Anonymous Function and a named function?
Anonymous functions exist only after they are called; whereas, Named functions to exist even if not called.
What are self Executing Functions?
These functions are executed right after its definition. Also called as Immediately Invoked Function Expressions (IIFE's). Syntax:
(function(){
console.log('in iffe');
})()
What is a function callback?
The callback function is a mechanism to send one function to another function as an argument; i.e., passing func as an argument to another function.
What happens when the recursion calling is applied on two functions?
The calling of recursion is possible in two functions, but the call comes to an end after some time.
Explain the term closure.
The inner functions can be called as closure when it has access to the outer function's variables.
Explain the terms synchronous and asynchronous code.
The synchronous code is something that should be finished before anything else can happen, or in other words, the synchronous code is blocking. And the Asynchronous code is something in which actions can happen and is not dependent on other actions- in other words, it is non-blocking.
Name the different types of pop up boxes in Javascript.
There are three types of pop up boxes in Javascript (i) alert() provides some information to the user with just an OK button (ii) confirm() asks a question to the user with two options Ok and cancel, and (iii) prompt() takes an input from the user.
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.
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);
}
Assuming d is an “empty” object in scope, say:
var d = {};
…what is accomplished using the following code?
[ 'zebra', 'horse' ].forEach(function(k) {
d[k] = undefined;
});
The snippet of code shown above sets two properties on the object d. Ideally, any lookup performed on a JavaScript object with an unset key evaluates to undefined. But running this code marks those properties as “own properties” of the object.
This is a useful strategy for ensuring that an object has a given set of properties. Passing this object to Object.keys will return an array with those set keys as well (even if their values are undefined).
What will the code below output to the console and why?
var arr1 = "john".split('');
var arr2 = arr1.reverse();
var arr3 = "jones".split('');
arr2.push(arr3);
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1));
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1));
The logged output will be:
"array 1: length=5 last=j,o,n,e,s"
"array 2: length=5 last=j,o,n,e,s"
arr1 and arr2 are the same (i.e. ['n','h','o','j', ['j','o','n','e','s'] ]) after the above code is executed for the following reasons:
Calling an array object’s 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).
The 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 a couple of side points here that can sometimes trip someone up in answering this question:
Passing an array to the 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).
Like Python, JavaScript honors negative subscripts in calls to array methods like 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.
How can you test the Ajax code?
JSUnit is the client side javascript code used as part of JUnit. JSUnit has been used for Ajax code.
Is Ajax said to be a technology platform or is it an architectural style?
Ajax supports both technology and as architectural style.
How can AJAX applications be debugged?
Two tools are used for debugging:
Fiddler for IE
Firebug for Mozilla.
What is Script Manager?
Script Manager helps manage the client side script of AJAX. Script Manager acts as a mediator as AJAX depends on JavaScript. Every page that uses AJAX has a Script Manager to enable AJAX libraries.
How Ajax objects can be created?
Following syntax can be used to create Ajax objects:
Var sample = New ajaxObject(‘path of the page’)
What are all the different data types that JSON supports?
JSON supports following data types:
String
Number
Boolean
Array
Object
Null.
There are many Javascript Frameworks available today, but the most commonly used frameworks are:
(i) Angular (ii) React (iii) Vue
Explain the typeof operator.
The operator typeof is an example of Unary Operators, which is used by placing it before its operand; which can be of any type.
What are anonymous functions in Javascript?
An anonymous function allows a developer to create a function that has no name. In other words, anonymous functions can be used to store a bit of functionality in a variable and pass that piece of functionality around.
What is the difference between an Anonymous Function and a named function?
Anonymous functions exist only after they are called; whereas, Named functions to exist even if not called.
 |
| Infosys JavaScript Most Frequently Asked Latest Interview Questions Answers |
What are self Executing Functions?
These functions are executed right after its definition. Also called as Immediately Invoked Function Expressions (IIFE's). Syntax:
(function(){
console.log('in iffe');
})()
What is a function callback?
The callback function is a mechanism to send one function to another function as an argument; i.e., passing func as an argument to another function.
What happens when the recursion calling is applied on two functions?
The calling of recursion is possible in two functions, but the call comes to an end after some time.
Explain the term closure.
The inner functions can be called as closure when it has access to the outer function's variables.
Explain the terms synchronous and asynchronous code.
The synchronous code is something that should be finished before anything else can happen, or in other words, the synchronous code is blocking. And the Asynchronous code is something in which actions can happen and is not dependent on other actions- in other words, it is non-blocking.
Name the different types of pop up boxes in Javascript.
There are three types of pop up boxes in Javascript (i) alert() provides some information to the user with just an OK button (ii) confirm() asks a question to the user with two options Ok and cancel, and (iii) prompt() takes an input from the user.
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.
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);
}
Assuming d is an “empty” object in scope, say:
var d = {};
…what is accomplished using the following code?
[ 'zebra', 'horse' ].forEach(function(k) {
d[k] = undefined;
});
The snippet of code shown above sets two properties on the object d. Ideally, any lookup performed on a JavaScript object with an unset key evaluates to undefined. But running this code marks those properties as “own properties” of the object.
This is a useful strategy for ensuring that an object has a given set of properties. Passing this object to Object.keys will return an array with those set keys as well (even if their values are undefined).
What will the code below output to the console and why?
var arr1 = "john".split('');
var arr2 = arr1.reverse();
var arr3 = "jones".split('');
arr2.push(arr3);
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1));
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1));
The logged output will be:
"array 1: length=5 last=j,o,n,e,s"
"array 2: length=5 last=j,o,n,e,s"
arr1 and arr2 are the same (i.e. ['n','h','o','j', ['j','o','n','e','s'] ]) after the above code is executed for the following reasons:
Calling an array object’s 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).
The 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 a couple of side points here that can sometimes trip someone up in answering this question:
Passing an array to the 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).
Like Python, JavaScript honors negative subscripts in calls to array methods like 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.
How can you test the Ajax code?
JSUnit is the client side javascript code used as part of JUnit. JSUnit has been used for Ajax code.
Is Ajax said to be a technology platform or is it an architectural style?
Ajax supports both technology and as architectural style.
How can AJAX applications be debugged?
Two tools are used for debugging:
Fiddler for IE
Firebug for Mozilla.
What is Script Manager?
Script Manager helps manage the client side script of AJAX. Script Manager acts as a mediator as AJAX depends on JavaScript. Every page that uses AJAX has a Script Manager to enable AJAX libraries.
How Ajax objects can be created?
Following syntax can be used to create Ajax objects:
Var sample = New ajaxObject(‘path of the page’)
What are all the different data types that JSON supports?
JSON supports following data types:
String
Number
Boolean
Array
Object
Null.
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