October 12, 2018

Sreekanth B

Dell EMC JavaScript Most Frequently Asked Latest Interview Questions Answers

What is AJAX Framework?

ASP.NET AJAX is a free framework to implement Ajax in asp.net web applications. It is used to quickly creating efficient and interactive Web applications that work across all browsers.

How can we cancel the XMLHttpRequest in AJAX?

Abort() method can be called to cancel the XMLHttpRequest in Ajax.

Is AJAX code cross browser compatible?

No, it is supporting cross browser compatible. If the browsers supports native XMLHttpRequest JavaScript object, then this can be used.

What is the name of object used for AJAX request?

XmlHttpRequest object is used for Ajax requests.

What is prerequisite for Update Panel in Ajax?

Script Manager is pre-requisite to use Update Panel controls.

How many update panel can be used per page?

There are no restrictions on the number of update panels per page.
Dell EMC JavaScript Most Frequently Asked Latest Interview Questions Answers
Dell EMC JavaScript Most Frequently Asked Latest Interview Questions Answers

How can you find out that an AJAX request has been completed?

ReadyState property is used to check whether AJAX request has been completed. If the property is equal to four, then the request has been completed and data is available.

Is javascript knowledge is required to do Ajax?

Yes, if you plan to develop new AJAX functionality for your web application.

What would the following lines of code output to the console?

console.log("0 || 1 = "+(0 || 1));
console.log("1 || 2 = "+(1 || 2));
console.log("0 && 1 = "+(0 && 1));
console.log("1 && 2 = "+(1 && 2));
Explain your answer.

The code will output the following four lines:

0 || 1 = 1
1 || 2 = 1
0 && 1 = 0
1 && 2 = 2
In JavaScript, both || and && are logical operators that return the first fully-determined “logical value” when evaluated from left to right.

The or (||) operator. In an expression of the form X||Y, X is first evaluated and interpreted as a boolean value. If this boolean value is true, then true (1) is returned and Y is not evaluated, since the “or” condition has already been satisfied. If this boolean value is “false”, though, we still don’t know if X||Y is true or false until we evaluate Y, and interpret it as a boolean value as well.

Accordingly, 0 || 1 evaluates to true (1), as does 1 || 2.

The and (&&) operator. In an expression of the form X&&Y, X is first evaluated and interpreted as a boolean value. If this boolean value is false, then false (0) is returned and Y is not evaluated, since the “and” condition has already failed. If this boolean value is “true”, though, we still don’t know if X&&Y is true or false until we evaluate Y, and interpret it as a boolean value as well.

However, the interesting thing with the && operator is that when an expression is evaluated as “true”, then the expression itself is returned. This is fine, since it counts as “true” in logical expressions, but also can be used to return that value when you care to do so. This explains why, somewhat surprisingly, 1 && 2 returns 2 (whereas you might it expect it to return true or 1).

What will be the output when the following code is executed? Explain.

console.log(false == '0')
console.log(false === '0')

The code will output:

true
false
In JavaScript, there are two sets of equality operators. The triple-equal operator === behaves like any traditional equality operator would: evaluates to true if the two expressions on either of its sides have the same type and the same value. The double-equal operator, however, tries to coerce the values before comparing them. It is therefore generally good practice to use the === rather than ==. The same holds true for !== vs !=.

What is the output out of the following code? Explain your answer.

var a={},
    b={key:'b'},
    c={key:'c'};

a[b]=123;
a[c]=456;

console.log(a[b]);

The output of this code will be 456 (not 123).

The reason for this is as follows: When setting an object property, JavaScript will implicitly stringify the parameter value. In this case, since b and c are both objects, they will both be converted to "[object Object]". As a result, a[b] anda[c] are both equivalent to a["[object Object]"] and can be used interchangeably. Therefore, setting or referencing a[c] is precisely the same as setting or referencing a[b].

What will the following code output to the console:

console.log((function f(n){return ((n > 1) ? n * f(n-1) : n)})(10));
Explain your answer.

The code will output the value of 10 factorial (i.e., 10!, or 3,628,800).

Here’s why:

The named function f() calls itself recursively, until it gets down to calling f(1) which simply returns 1. Here, therefore, is what this does:

f(1): returns n, which is 1
f(2): returns 2 * f(1), which is 2
f(3): returns 3 * f(2), which is 6
f(4): returns 4 * f(3), which is 24
f(5): returns 5 * f(4), which is 120
f(6): returns 6 * f(5), which is 720
f(7): returns 7 * f(6), which is 5040
f(8): returns 8 * f(7), which is 40320
f(9): returns 9 * f(8), which is 362880
f(10): returns 10 * f(9), which is 3628800

Consider the code snippet below. What will the console output be and why?

(function(x) {
    return (function(y) {
        console.log(x);
    })(2)
})(1);

The output will be 1, even though the value of x is never set in the inner function. Here’s why:

As explained in our JavaScript Hiring Guide, a closure is a function, along with all variables or functions that were in-scope at the time that the closure was created. In JavaScript, a closure is implemented as an “inner function”; i.e., a function defined within the body of another function. An important feature of closures is that an inner function still has access to the outer function’s variables.

Therefore, in this example, since x is not defined in the inner function, the scope of the outer function is searched for a defined variable x, which is found to have a value of 1.

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