Computer Sciences Corporation JavaScript Interview Questions

Computer Sciences Corporation JavaScript Most Frequently Asked Interview Questions Answers

Can we use nested update panel in Ajax?

Yes, we can use nested update panel in Ajax. Update panels can be nested to have more control over the Page Refresh.

What are the types of post back in Ajax?

There are two types of post backs:

Synchronous Postback
Asynchronous Postback

How can we handle exception handling in Ajax?

ErrorTemplate which is the child tag of Script Manager is used to handle exception handling in Ajax.

What are the components of the ASP.NET Ajax Client Library?

Following components are used in Ajax client library:

Component Layer
Core Services Layer
Browser Compatibility Layer

Computer Sciences Corporation JavaScript Most Frequently Asked Interview Questions Answers

What are the controls of the Script Management group?

The controls of script Management group are:

ScriptManager
ScriptManagerProxy

 How many types of ready states in Ajax?

There are four ready states in Ajax:

Initialization
Request
Process
Ready

What is the difference between RegisterClientScriptBlock, RegisterClientScriptInclude and RegisterClientScriptResource?

Following are the functions:

RegisterClientScriptBlock – The script is specified as a string parameter.
RegisterClientScriptInclude – By setting the source attribute to a URL that point to a script file.
RegisterClientScriptResource – specifies Resource name in an assembly. The source attribute is automatically populated with a URL by a call to an HTTP handler that retrieves the named script from the assembly.

How do you clone an object?

var obj = {a: 1 ,b: 2}
var objclone = Object.assign({},obj);
Now the value of objclone is {a: 1 ,b: 2} but points to a different object than obj.

Note the potential pitfall, though: Object.clone() will just do a shallow copy, not a deep copy. This means that nested objects aren’t copied. They still refer to the same nested objects as the original:

let obj = {
    a: 1,
    b: 2,
    c: {
        age: 30
    }
};

var objclone = Object.assign({},obj);
console.log('objclone: ', objclone);

obj.c.age = 45;
console.log('After Change - obj: ', obj);           // 45 - This also changes
console.log('After Change - objclone: ', objclone); // 45

for (let i = 0; i < 5; i++) {
  setTimeout(function() { console.log(i); }, i * 1000 );
}
What will this code print?

It will print 0 1 2 3 4, because we use let instead of var here. The variable i is only seen in the for loop’s block scope.

What do the following lines output, and why?

console.log(1 < 2 < 3);
console.log(3 > 2 > 1);

The first statement returns true which is as expected.

The second returns false because of how the engine works regarding operator associativity for < and >. It compares left to right, so 3 > 2 > 1 JavaScript translates to true > 1. true has value 1, so it then compares 1 > 1, which is false.

How do you add an element at the begining of an array? How do you add one at the end?

var myArray = ['a', 'b', 'c', 'd'];
myArray.push('end');
myArray.unshift('start');
console.log(myArray); // ["start", "a", "b", "c", "d", "end"]
With ES6, one can use the spread operator:

myArray = ['start', ...myArray];
myArray = [...myArray, 'end'];
Or, in short:

myArray = ['start', ...myArray, 'end'];

Imagine you have this code:

var a = [1, 2, 3];
a) Will this result in a crash?

a[10] = 99;
b) What will this output?

console.log(a[6]);

a) It will not crash. The JavaScript engine will make array slots 3 through 9 be “empty slots.”

b) Here, a[6] will output undefined, but the slot still remains empty rather than filled with undefined. This may be an important nuance in some cases. For example, when using map(), empty slots will remain empty in map()’s output, but undefined slots will be remapped using the function passed to it:

var b = [undefined];
b[2] = 1;
console.log(b);             // (3) [undefined, empty × 1, 1]
console.log(b.map(e => 7)); // (3) [7,         empty × 1, 7]

What is the value of typeof undefined == typeof NULL?

The expression will be evaluated to true, since NULL will be treated as any other undefined variable.

Note: JavaScript is case-sensitive and here we are using NULL instead of null.

What would following code return?

console.log(typeof typeof 1);

string

typeof 1 will return "number" and typeof "number" will return string.