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.
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 are DOM utilized in JavaScript?
DOM stands for Document Object Model and is responsible for how various objects in a document interact with each other. DOM is required for developing web pages, which includes objects like paragraph, links, etc. These objects can be operated to include actions like add or delete. DOM is also required to add extra capabilities to a web page. On top of that, the use of API gives an advantage over other existing models.
How are event handlers utilized in JavaScript?
Events are the actions that result from activities, such as clicking a link or filling a form, by the user. An event handler is required to manage proper execution of all these events. Event handlers are an extra attribute of the object. This attribute includes event’s name and the action taken if the event takes place.
Explain the role of deferred scripts in JavaScript?
By default, the parsing of the HTML code, during page loading, is paused until the script has not stopped executing. It means, if the server is slow or the script is particularly heavy, then the webpage is displayed with a delay. While using Deferred, scripts delays execution of the script till the time HTML parser is running. This reduces the loading time of web pages and they get displayed faster.
What are the various functional components in JavaScript?
The different functional components in JavaScript are-
First-class functions: Functions in JavaScript are utilized as first class objects. This usually means that these functions can be passed as arguments to other functions, returned as values from other functions, assigned to variables or can also be stored in data structures.
Nested functions: The functions, which are defined inside other functions, are called Nested functions. They are called ‘everytime’ the main function is invoked.
Write about the errors shown in JavaScript?
JavaScript gives a message if it encounters an error. The recognized errors are –
Load-time errors: The errors shown at the time of the page loading are counted under Load-time errors. These errors are encountered by the use of improper syntax, and thus are detected while the page is getting loaded.
Run-time errors: This is the error that comes up while the program is running. It is caused by illegal operations, for example, division of a number by zero, or trying to access a non-existent area of the memory.
Logic errors: It is caused by the use of syntactically correct code, which does not fulfill the required task. For example, an infinite loop.
What are Screen objects?
Screen objects are used to read the information from the client’s screen. The properties of screen objects are –
AvalHeight: Gives the height of client’s screen
AvailWidth: Gives the width of client’s screen.
ColorDepth: Gives the bit depth of images on the client’s screen
Height: Gives the total height of the client’s screen, including the taskbar
Width: Gives the total width of the client’s screen, including the taskbar
Explain the unshift() method ?
This method is functional at the starting of the array, unlike the push(). It adds the desired number of elements to the top of an array. For example –
var name = [ "john" ];
name.unshift( "charlie" );
name.unshift( "joseph", "Jane" );
console.log(name);
The output is shown below:
[" Jane ", " joseph ", " charlie ", " john "]
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.
What will the code below output to the console and why ?
console.log(1 + "2" + "2");
console.log(1 + +"2" + "2");
console.log(1 + -"1" + "2");
console.log(+"1" + "1" + "2");
console.log( "A" - "B" + "2");
console.log( "A" - "B" + 2);
The above code will output the following to the console:
"122"
"32"
"02"
"112"
"NaN2"
NaN
Here’s why…
The fundamental issue here is that JavaScript (ECMAScript) is a loosely typed language and it performs automatic type conversion on values to accommodate the operation being performed. Let’s see how this plays out with each of the above examples.
Example 1: 1 + "2" + "2" Outputs: "122" Explanation: The first operation to be performed in 1 + "2". Since one of the operands ("2") is a string, JavaScript assumes it needs to perform string concatenation and therefore converts the type of 1 to "1", 1 + "2" yields "12". Then, "12" + "2" yields "122".
Example 2: 1 + +"2" + "2" Outputs: "32" Explanation: Based on order of operations, the first operation to be performed is +"2" (the extra + before the first "2" is treated as a unary operator). Thus, JavaScript converts the type of "2" to numeric and then applies the unary + sign to it (i.e., treats it as a positive number). As a result, the next operation is now 1 + 2 which of course yields 3. But then, we have an operation between a number and a string (i.e., 3 and "2"), so once again JavaScript converts the type of the numeric value to a string and performs string concatenation, yielding "32".
Example 3: 1 + -"1" + "2" Outputs: "02" Explanation: The explanation here is identical to the prior example, except the unary operator is - rather than +. So "1" becomes 1, which then becomes -1 when the - is applied, which is then added to 1 yielding 0, which is then converted to a string and concatenated with the final "2" operand, yielding "02".
Example 4: +"1" + "1" + "2" Outputs: "112" Explanation: Although the first "1" operand is typecast to a numeric value based on the unary + operator that precedes it, it is then immediately converted back to a string when it is concatenated with the second "1" operand, which is then concatenated with the final "2" operand, yielding the string "112".
Example 5: "A" - "B" + "2" Outputs: "NaN2" Explanation: Since the - operator can not be applied to strings, and since neither "A" nor "B" can be converted to numeric values, "A" - "B" yields NaN which is then concatenated with the string "2" to yield “NaN2”.
Example 6: "A" - "B" + 2 Outputs: NaN Explanation: As exlained in the previous example, "A" - "B" yields NaN. But any operator applied to NaN with any other numeric operand will still yield NaN.
DOM stands for Document Object Model and is responsible for how various objects in a document interact with each other. DOM is required for developing web pages, which includes objects like paragraph, links, etc. These objects can be operated to include actions like add or delete. DOM is also required to add extra capabilities to a web page. On top of that, the use of API gives an advantage over other existing models.
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| Mahindra Satyam JavaScript Most Frequently Asked Latest Interview Questions Answers |
How are event handlers utilized in JavaScript?
Events are the actions that result from activities, such as clicking a link or filling a form, by the user. An event handler is required to manage proper execution of all these events. Event handlers are an extra attribute of the object. This attribute includes event’s name and the action taken if the event takes place.
Explain the role of deferred scripts in JavaScript?
By default, the parsing of the HTML code, during page loading, is paused until the script has not stopped executing. It means, if the server is slow or the script is particularly heavy, then the webpage is displayed with a delay. While using Deferred, scripts delays execution of the script till the time HTML parser is running. This reduces the loading time of web pages and they get displayed faster.
What are the various functional components in JavaScript?
The different functional components in JavaScript are-
First-class functions: Functions in JavaScript are utilized as first class objects. This usually means that these functions can be passed as arguments to other functions, returned as values from other functions, assigned to variables or can also be stored in data structures.
Nested functions: The functions, which are defined inside other functions, are called Nested functions. They are called ‘everytime’ the main function is invoked.
Write about the errors shown in JavaScript?
JavaScript gives a message if it encounters an error. The recognized errors are –
Load-time errors: The errors shown at the time of the page loading are counted under Load-time errors. These errors are encountered by the use of improper syntax, and thus are detected while the page is getting loaded.
Run-time errors: This is the error that comes up while the program is running. It is caused by illegal operations, for example, division of a number by zero, or trying to access a non-existent area of the memory.
Logic errors: It is caused by the use of syntactically correct code, which does not fulfill the required task. For example, an infinite loop.
What are Screen objects?
Screen objects are used to read the information from the client’s screen. The properties of screen objects are –
AvalHeight: Gives the height of client’s screen
AvailWidth: Gives the width of client’s screen.
ColorDepth: Gives the bit depth of images on the client’s screen
Height: Gives the total height of the client’s screen, including the taskbar
Width: Gives the total width of the client’s screen, including the taskbar
Explain the unshift() method ?
This method is functional at the starting of the array, unlike the push(). It adds the desired number of elements to the top of an array. For example –
var name = [ "john" ];
name.unshift( "charlie" );
name.unshift( "joseph", "Jane" );
console.log(name);
The output is shown below:
[" Jane ", " joseph ", " charlie ", " john "]
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.
What will the code below output to the console and why ?
console.log(1 + "2" + "2");
console.log(1 + +"2" + "2");
console.log(1 + -"1" + "2");
console.log(+"1" + "1" + "2");
console.log( "A" - "B" + "2");
console.log( "A" - "B" + 2);
The above code will output the following to the console:
"122"
"32"
"02"
"112"
"NaN2"
NaN
Here’s why…
The fundamental issue here is that JavaScript (ECMAScript) is a loosely typed language and it performs automatic type conversion on values to accommodate the operation being performed. Let’s see how this plays out with each of the above examples.
Example 1: 1 + "2" + "2" Outputs: "122" Explanation: The first operation to be performed in 1 + "2". Since one of the operands ("2") is a string, JavaScript assumes it needs to perform string concatenation and therefore converts the type of 1 to "1", 1 + "2" yields "12". Then, "12" + "2" yields "122".
Example 2: 1 + +"2" + "2" Outputs: "32" Explanation: Based on order of operations, the first operation to be performed is +"2" (the extra + before the first "2" is treated as a unary operator). Thus, JavaScript converts the type of "2" to numeric and then applies the unary + sign to it (i.e., treats it as a positive number). As a result, the next operation is now 1 + 2 which of course yields 3. But then, we have an operation between a number and a string (i.e., 3 and "2"), so once again JavaScript converts the type of the numeric value to a string and performs string concatenation, yielding "32".
Example 3: 1 + -"1" + "2" Outputs: "02" Explanation: The explanation here is identical to the prior example, except the unary operator is - rather than +. So "1" becomes 1, which then becomes -1 when the - is applied, which is then added to 1 yielding 0, which is then converted to a string and concatenated with the final "2" operand, yielding "02".
Example 4: +"1" + "1" + "2" Outputs: "112" Explanation: Although the first "1" operand is typecast to a numeric value based on the unary + operator that precedes it, it is then immediately converted back to a string when it is concatenated with the second "1" operand, which is then concatenated with the final "2" operand, yielding the string "112".
Example 5: "A" - "B" + "2" Outputs: "NaN2" Explanation: Since the - operator can not be applied to strings, and since neither "A" nor "B" can be converted to numeric values, "A" - "B" yields NaN which is then concatenated with the string "2" to yield “NaN2”.
Example 6: "A" - "B" + 2 Outputs: NaN Explanation: As exlained in the previous example, "A" - "B" yields NaN. But any operator applied to NaN with any other numeric operand will still yield NaN.
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