Amadeus IT Group Most Frequently Asked Latest JavaScript Interview Questions Answers

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

Amadeus IT Group Most Frequently Asked Latest JavaScript Interview Questions Answers

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 is a Typed Language?

Typed Language is in which the values are associated with values and not with variables. It is of two types:

Dynamically: in this, the variable can hold multiple types; like in JS a variable can take number, chars.

Statically: in this, the variable can hold only one type, like in Java a variable declared of string can take only set of characters and nothing else.

NOTE: Typescript is a superset of JS which is of typed language.

What does a typeof operator do?

The operator typeof gives the type of a variable/data available in it. The typeof operator can be used on Reference data types also.

Name some of the ways in which Type Conversion is possible.

Type Conversion is to convert from one data type to another data type.

(i) Number to String Conversion.

toString: Other data type to String.
val = (5).toString(); converts integer to string.
val = (true).toString(); converts boolean to string.
Convert from Number/Boolean/Date to String.
Number to String: String(9) converts num to string
Boolean to String: String(true) converts bool to str
Date to String: String(new Date()) date to string
Array to String: String([2,2,2]) Array to string.

(ii) String to Number Conversion.

parseInt: String to Int only (no decimals)
val = parseInt('11'); outputs 100 as number
val = parseFloat('22.22') outputs 22.22 as float
Convert from String/Boolean to Number/Date.
String to Number: Number('9') converts str - num
(9).toFixed(4) gives 9.0000 as the output
Boolean to Number: Number(true) converts to no.
Null to Number: Number(null) converts to no. (0)
Chars to Number: Number('ss') give NaN.

What is the difference between Local Storage and Session Storage?

Local Storage will stay until it is manually cleared through settings or program.

Session Storage will leave when the browser is closed.

What is the difference between the keywords var and let?

The keyword var is from the beginning of Javascript; whereas, let is introduced in ES2015/ES6. The keyword let has a block scope; whereas, the keyword var has a functional scope.

Are Typescript and Javascript the same?

Typescript is not the next version of Javascript but is developed by Microsoft and can be taken as a superset to Javascript; the code written in Typescript is later compiled into Javascript. Typescript adds new features like Interfaces, Generics, etc.

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');
})()

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.

What will the following code output and why?

var b = 1;
function outer(){
    var b = 2
    function inner(){
        b++;
        var b = 3;
        console.log(b)
    }
    inner();
}
outer();

Output to the console will be “3”.

There are three closures in the example, each with it’s own var b declaration. When a variable is invoked closures will be checked in order from local to global until an instance is found. Since the inner closure has a b variable of its own, that is what will be output.

Furthermore, due to hoisting the code in inner will be interpreted as follows:

function inner () {
    var b; // b is undefined
    b++; // b is NaN
    b = 3; // b is 3
    console.log(b); // output "3"
}

How to control the duration of an Ajax request?

AsyncPostBackTimeout property is used to control the duration of Ajax request. Deafult value of this property is 90 seconds.

Example –

<asp:ScriptManager runat=”server” id=”sample” AsyncPostBackTimeout=”40”/>
1
<asp:ScriptManager runat=”server” id=”sample” AsyncPostBackTimeout=”40”/>

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 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.