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Programming in JavaScript

Q. Predict the output of the following JavaScript code?

console.log(+'meow'); // Output: NaN

Q. Predict the output of the following JavaScript code?

var result;
for (var i = 5; i > 0; i--) {
    result = result + i;
}
console.log(result); // Output: NaN

Q. Predict the output of the following JavaScript code?

var a = 1.2;
console.log(typeof a); // Output: Number

Q. Predict the output of the following JavaScript code?

var x = 10;
if (x) {
    let x = 4;
}
console.log(x); // Output: 10

Q. Predict the output of the following JavaScript code?

console.log(0.1 + 0.2 == 0.3); // Output: false

Q. Predict the output of the following JavaScript code?

console.log(1 + -'1' + 2); // Output: 2

Q. Predict the output of the following JavaScript code?

(function(x) {
    return (function(y) {
    console.log(x);
    })(10);
})(20);   // Output: 20

Q. Predict the output of the following JavaScript code?

var num = 20;
var getNumber = function() {
    console.log(num);
    var num = 10;
};
getNumber(); // Output: undefined

Q. Predict the output of the following JavaScript code?

function f1() {
    num = 10;
}
f1();
console.log('window.num: '+window.num); // output: 10

Q. Predict the output of the following JavaScript code?

console.log('(null + undefined): '+(null + undefined)); // Output: NaN

Q. Predict the output of the following JavaScript code?

(function() {
    var a = (b = 3);
})();

console.log('value of a : ' + a); // Output: undefined
console.log('value of b : ' + b); // Output: 3

Q. Predict the output of the following JavaScript code?

var y = 1;
if (function f() {}) {
    y += typeof f;
}
console.log(y); // Output: 1Object

Q. Predict the output of the following JavaScript code?

var k = 1;
if (1) {
    eval(function foo() {});
    k += typeof foo;
}
console.log(k); // Output: 1undefined

Q. Predict the output of the following JavaScript code?

var k = 1;
if (1) {
    function foo() {}
    k += typeof foo;
}
console.log(k); // Output: 1function

Q. Predict the output of the following JavaScript code?

console.log('(-1 / 0): '+(-1 / 0)); // Output: -Infinity
console.log('(1 / 0): '+(1 / 0));   // Output: Infinity
console.log('(0 / 0): '+(0 / 0));   // Output: NaN
console.log('(0 / 1): '+(0 / 1));   // Output: 0

Q. Predict the output of the following JavaScript code?

var a = 4;
var b = '5';
var c = 6;

console.log('(a + b): '+(a + b)); // Output: 45
console.log('(a - b): '+(a - b)); // Output: -1
console.log('(a * b): '+(a * b)); // Output: 20
console.log('(a / b): '+(a / b)); // Output: 0.8
console.log('(a % b): '+(a % b)); // Output: 4

Q. Predict the output of the following JavaScript code?

console.log("MAX : " + Math.max(10, 2, NaN));  // Output: NaN
console.log("MAX : " + Math.max());            // Output: -Infinity

Q. Predict the output of the following JavaScript code?

(function(){
    var a = b = 3;
})();

console.log("a defined? " + (typeof a !== 'undefined')); // Output: true
console.log("b defined? " + (typeof b !== 'undefined')); // Output: true

Q. Predict the output of the following JavaScript code?

var myObject = {
    foo: "bar",
    func: function() {
        var self = this;
        console.log("outer func:  this.foo = " + this.foo); // Output: this.foo = bar
        console.log("outer func:  self.foo = " + self.foo); // Output: self.foo = bar
        (function() {
            console.log("inner func:  this.foo = " + this.foo); // Output: this.foo = function foo() {}
            console.log("inner func:  self.foo = " + self.foo); // Output: self.foo = bar
        }());
    }
};
myObject.func();

Q. Predict the output of the following JavaScript code?

console.log(0.1 + 0.2);        // Output: 0.30000000000000004
console.log(0.1 + 0.2 == 0.3); // Output: false

Q. Predict the output of the following JavaScript code?

(function() {
    console.log(1); 
    setTimeout(function(){console.log(2)}, 1000); 
    setTimeout(function(){console.log(3)}, 0); 
    console.log(4);
})();
// Output: 1, 4, 3, 2

Q. Predict the output of the following JavaScript code?

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)); //array 1: length=5 last=j,o,n,e,s
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1)); //array 2: length=5 last=j,o,n,e,s

Q. Predict the output of the following JavaScript code?

console.log(1 +  "2" + "2");    // Output: 122
console.log(1 +  +"2" + "2");   // Output: 32
console.log(1 +  -"1" + "2");   // Output: 02
console.log(+"1" +  "1" + "2"); // Output: 112
console.log( "A" - "B" + "2");  // Output: NaN2
console.log( "A" - "B" + 2);    // Output: NaN

Q. Predict the output of the following JavaScript code?

for (var i = 0; i < 5; i++) {
    setTimeout(function() { console.log(i); }, i * 1000 );
}
// Output: 145, 5, 5, 5, 5, 5

Q. Predict the output of the following JavaScript code?

for (var i = 0; i < 5; i++) {
    (function(x) {
        setTimeout(function() { console.log(x); }, x * 1000 );
    })(i);
}
//Output:- 0, 1, 2, 3, 4

Q. Predict the output of the following JavaScript code?

console.log("0 || 1 = "+(0 || 1)); // Output: 1
console.log("1 || 2 = "+(1 || 2)); // Output: 1
console.log("0 && 1 = "+(0 && 1)); // Output: 0
console.log("1 && 2 = "+(1 && 2)); // Output: 2

Q. Predict the output of the following JavaScript code?

console.log(false == '0')  // Output: true
console.log(false === '0') // Output: false

Q. Predict the output of the following JavaScript code?

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

a[b]=123;
a[c]=456;
console.log(a[b]); // Output: 456

Q. Predict the output of the following JavaScript code?

console.log((function f(n){return ((n > 1) ? n * f(n-1) : n)})(10)); // Output: 3628800

Q. Predict the output of the following JavaScript code?

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

Q. Predict the output of the following JavaScript code?

var hero = {
    _name: 'John Doe',
    getSecretIdentity: function (){
        return this._name;
    }
};
var stoleSecretIdentity = hero.getSecretIdentity;

console.log(stoleSecretIdentity());    // Output: undefined
console.log(hero.getSecretIdentity()); // Output: John Doe

Q. Predict the output of the following JavaScript code?

var length = 10;
function fn() {
    console.log(this.length); 
}

var obj = {
    length: 5,
    method: function(fn) {
    fn();
    arguments[0]();
    }
};

obj.method(fn, 1);
//Output: 10, 2

Q. Predict the output of the following JavaScript code?

(function () {
    try {
        throw new Error();
    } catch (x) {
        var x = 1, y = 2;
        console.log(x);
    }
    console.log(x);
    console.log(y);
})();
//Output:  1, undefined, 2

Q. Predict the output of the following JavaScript code?

var x = 21;
var girl = function () {
    console.log(x); // Output: undefined
    var x = 20;
};
girl ();

Q. Predict the output of the following JavaScript code?

console.log(1 < 2 < 3); // Output: true
console.log(3 > 2 > 1); // Output: false

Q. Predict the output of the following JavaScript code?

console.log(typeof typeof 1); // Output: string

Q. Predict the output of the following JavaScript code?

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

Q. Hoisting example in javascript

x = 10
console.log(x);
var x; // Output: 10

Q. Predict the output of the following JavaScript code?

const arr = [1, 2];
arr.push(3);       // Output: 1, 2, 3

Q. Predict the output of the following JavaScript code?

var o = new F(); 
o.constructor === F;

Q. Predict the output of the following JavaScript code?

let sum = (a, b) => { a + b;}; 
console.log(sum(10, 20)); // Output: undefined; return keyword is missing

Q. Predict the output of the following JavaScript code?

var arr = ['javascript', 'typescript', 'es6']; 
var searchValue = (value) => {  
        return arr.filter(item => {    
        return item. indexOf(value) > -1; 
        }); 
} 
console.log(searchValue ('script'));

Q. Write the program using fatarrow function?

var a = [1, 2, 3, 4]; 
function sumUsingFunction (acc, value) {   
  return acc + value; 
} 
var sumOfArrayUsingFunc = a.reduce(sumUsingFunc);

Q. Write a program that prints the numbers from 1 to 15. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”?

for (var i=1; i <= 15; i++){
    if (i % 15 == 0) console.log("FizzBuzz");
    else if (i % 3 == 0) console.log("Fizz");
    else if (i % 5 == 0) console.log("Buzz");
    else console.log(i);
}

Output:

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz

Solution - 02

for (var i = 1; i <= 15; i++) {
  var f = i % 3 == 0, b = i % 5 == 0;
  console.log(f ? b ? "FizzBuzz" : "Fizz" : b ? "Buzz" : i);
}

Q. What will be the output of the following code?

var output = (function(x) {
  delete x;
  return x;
})(0);

console.log(output);

The code above will output 0 as output. delete operator is used to delete a property from an object. Here x is not an object it's local variable. delete operator doesn't affect local variables.

Q. What will be the output of the following code?

var x = 1;
var output = (function() {
  delete x;
  return x;
})();

console.log(output);

The code above will output 1 as output. delete operator is used to delete a property from an object. Here x is not an object it's global variable of type number.

Q. What will be the output of the following code?

var x = { foo : 1};
var output = (function() {
  delete x.foo;
  return x.foo;
})();

console.log(output);

The code above will output undefined as output. delete operator is used to delete a property from an object. Here x is an object which has foo as a property and from a self-invoking function, we are deleting the foo property of object x and after deletion, we are trying to reference deleted property foo which result undefined.

Q. What will be the output of the following code?

var Employee = {
  company: 'xyz'
}
var emp1 = Object.create(Employee);
delete emp1.company
console.log(emp1.company);

The code above will output xyz as output. Here emp1 object got company as prototype property. delete operator doesn't delete prototype property.

emp1 object doesn't have company as its own property. you can test it console.log(emp1.hasOwnProperty('company')); //output : false However, we can delete company property directly from Employee object using delete Employee.company or we can also delete from emp1 object using __proto__ property delete emp1.__proto__.company.

Q. What will be the output of the following code?

var trees = ["xyz", "xxxx", "test", "ryan", "apple"];
delete trees[3];
console.log(trees.length);

The code above will output 5 as output. When we used delete operator for deleting an array element then, the array length is not affected by this. This holds even if you deleted all elements of an array using delete operator.

So when delete operator removes an array element that deleted element is no longer present in the array. In place of value at deleted index undefined x 1 in chrome and undefined is placed at the index. If you do console.log(trees) output ["xyz", "xxxx", "test", undefined × 1, "apple"] in Chrome and in Firefox ["xyz", "xxxx", "test", undefined, "apple"].

Q. What will be the output of the following code?

var bar = true;
console.log(bar + 0);   
console.log(bar + "xyz");  
console.log(bar + true);  
console.log(bar + false);

The code above will output 1, "truexyz", 2, 1 as output. Here's a general guideline for the plus operator:

  • Number + Number -> Addition
  • Boolean + Number -> Addition
  • Boolean + Boolean -> Addition
  • Number + String -> Concatenation
  • String + Boolean -> Concatenation
  • String + String -> Concatenation

Q. What will be the output of the following code?

var z = 1, y = z = typeof y;
console.log(y);

The code above will print string "undefined" as output. According to associativity rule operator with the same precedence are processed based on their associativity property of operator. Here associativity of the assignment operator is Right to Left so first typeof y will evaluate first which is string "undefined" and assigned to z and then y would be assigned the value of z. The overall sequence will look like that:

var z;
z = 1;
var y;
z = typeof y;
y = z;

Q. What will be the output of the following code?

// NFE (Named Function Expression)
var foo = function bar() { return 12; };
typeof bar();

The output will be Reference Error. To fix the bug we can try to rewrite the code a little bit:

Sample 1

var bar = function() { return 12; };
typeof bar();

or

Sample 2

function bar() { return 12; };
typeof bar();

The function definition can have only one reference variable as a function name, In sample 1 bar is reference variable which is pointing to anonymous function and in sample 2 we have function statement and bar is the function name.

var foo = function bar() {
  // foo is visible here
  // bar is visible here
  console.log(typeof bar()); // Works here :)
};
// foo is visible here
// bar is undefined here

Q. What is the output of the following?

bar();
(function abc(){console.log('something')})();
function bar(){console.log('bar got called')};

The output will be :

bar got called
something

Since the function is called first and defined during parse time the JS engine will try to find any possible parse time definitions and start the execution loop which will mean function is called first even if the definition is post another function.

Q. What will be the output of the following code?

var salary = "1000$";

(function () {
  console.log("Original salary was " + salary);

  var salary = "5000$";

  console.log("My New Salary " + salary);
})();

The code above will output: undefined, 5000$ because of hoisting. In the code presented above, you might be expecting salary to retain it values from outer scope until the point that salary was re-declared in the inner scope. But due to hoisting salary value was undefined instead. To understand it better have a look of the following code, here salary variable is hoisted and declared at the top in function scope. When we print its value using console.log the result is undefined. Afterwards the variable is redeclared and the new value "5000$" is assigned to it.

var salary = "1000$";

(function () {
  var salary = undefined;
  console.log("Original salary was " + salary);

  salary = "5000$";

  console.log("My New Salary " + salary);
})();

Q. What would be the output of the following code?

function User(name) {
  this.name = name || "JsGeeks";
}

var person = new User("xyz")["location"] = "USA";
console.log(person);

The output of above code would be "USA". Here new User("xyz") creates a brand new object and created property location on that and USA has been assigned to object property location and that has been referenced by the person.

Let say new User("xyz") created a object called foo. The value "USA" will be assigned to foo["location"], but according to ECMAScript Specification , pt 12.14.4 the assignment will itself return the rightmost value: in our case it's "USA". Then it will be assigned to person.

To better understand what's going on here, try to execute this code in console, line by line:

function User(name) {
 this.name = name || "JsGeeks";
}

var person;
var foo = new User("xyz");
foo["location"] = "USA";
// the console will show you that the result of this is "USA"

Q. What would be the output of following code?

var strA = "hi there";
var strB = strA;
strB="bye there!";
console.log (strA)

The output will 'hi there' because we're dealing with strings here. Strings are passed by value, that is, copied.

Q. What would be the output of following code?

var objA = {prop1: 42};
var objB = objA; 
objB.prop1 = 90;
console.log(objA)

The output will {prop1: 90} because we're dealing with objects here. Objects are passed by reference, that is, objA and objB point to the same object in memory.

Q. What would be the output of following code?

var objA = {prop1: 42};
var objB = objA;
objB = {};
console.log(objA)

The output will {prop1: 42}.

When we assign objA to objB, the objB variable will point to the same object as the objB variable.

However, when we reassign objB to an empty object, we simply change where objB variable references to. This doesn't affect where objA variable references to.

Q. What would be the output of following code?

var arrA = [0,1,2,3,4,5];
var arrB = arrA;
arrB[0]=42;
console.log(arrA)

The output will be [42,1,2,3,4,5].

Arrays are object in JavaScript and they are passed and assigned by reference. This is why both arrA and arrB point to the same array [0,1,2,3,4,5]. That's why changing the first element of the arrB will also modify arrA: it's the same array in the memory.

Q. What would be the output of following code?

var arrA = [0,1,2,3,4,5];
var arrB = arrA.slice();
arrB[0]=42;
console.log(arrA)

The output will be [0,1,2,3,4,5].

The slice function copies all the elements of the array returning the new array. That's why arrA and arrB reference two completely different arrays.

Q. What would be the output of following code?

var arrA = [{prop1: "value of array A!!"},  {someProp: "also value of array A!"}, 3,4,5];
var arrB = arrA;
arrB[0].prop1=42;
console.log(arrA);

The output will be [{prop1: 42}, {someProp: "also value of array A!"}, 3,4,5].

Arrays are object in JS, so both varaibles arrA and arrB point to the same array. Changing arrB[0] is the same as changing arrA[0]

Q. What would be the output of following code?

var arrA = [{prop1: "value of array A!!"}, {someProp: "also value of array A!"},3,4,5];
var arrB = arrA.slice();
arrB[0].prop1=42;
arrB[3] = 20;
console.log(arrA);

The output will be [{prop1: 42}, {someProp: "also value of array A!"}, 3,4,5].

The slice function copies all the elements of the array returning the new array. However, it doesn't do deep copying. Instead it does shallow copying. You can imagine slice implemented like this:

function slice(arr) {
   var result = [];
   for (i = 0; i< arr.length; i++) {
       result.push(arr[i]);
   }
   return result; 
}

Look at the line with result.push(arr[i]). If arr[i] happens to be a number or string, it will be passed by value, in other words, copied. If arr[i] is an object, it will be passed by reference.

In case of our array arr[0] is an object {prop1: "value of array A!!"}. Only the reference to this object will be copied. This effectively means that arrays arrA and arrB share first two elements.

This is why changing the property of arrB[0] in arrB will also change the arrA[0].

Q. console.log(employeeId);

  1. Some Value
  2. Undefined
  3. Type Error
  4. ReferenceError: employeeId is not defined

Answer: 4) ReferenceError: employeeId is not defined

Q. What would be the output of following code?

console.log(employeeId);
var employeeId = '19000';
  1. Some Value
  2. undefined
  3. Type Error
  4. ReferenceError: employeeId is not defined

Answer: 2) undefined

Q. What would be the output of following code?

var employeeId = '1234abe';
(function(){
	console.log(employeeId);
	var employeeId = '122345';
})();
  1. '122345'
  2. undefined
  3. Type Error
  4. ReferenceError: employeeId is not defined

Answer: 2) undefined

Q. What would be the output of following code?

var employeeId = '1234abe';
(function() {
	console.log(employeeId);
	var employeeId = '122345';
	(function() {
		var employeeId = 'abc1234';
	}());
}());
  1. '122345'
  2. undefined
  3. '1234abe'
  4. ReferenceError: employeeId is not defined

Answer: 2) undefined

Q. What would be the output of following code?

(function() {
	console.log(typeof displayFunc);
	var displayFunc = function(){
		console.log("Hi I am inside displayFunc");
	}
}());
  1. undefined
  2. function
  3. 'Hi I am inside displayFunc'
  4. ReferenceError: displayFunc is not defined

Answer: 1) undefined

Q. What would be the output of following code?

var employeeId = 'abc123';
function foo(){
	employeeId = '123bcd';
	return;
}
foo();
console.log(employeeId);
  1. undefined
  2. '123bcd'
  3. 'abc123'
  4. ReferenceError: employeeId is not defined

Answer: 2) '123bcd'

Q. What would be the output of following code?

var employeeId = 'abc123';

function foo() {
	employeeId = '123bcd';
	return;

	function employeeId() {}
}
foo();
console.log(employeeId);
  1. undefined
  2. '123bcd'
  3. 'abc123'
  4. ReferenceError: employeeId is not defined

Answer: 3) 'abc123'

Q. What would be the output of following code?

var employeeId = 'abc123';

function foo() {
	employeeId();
	return;

	function employeeId() {
		console.log(typeof employeeId);
	}
}
foo();
  1. undefined
  2. function
  3. string
  4. ReferenceError: employeeId is not defined

Answer: 2) 'function'

Q. What would be the output of following code?

function foo() {
	employeeId();
	var product = 'Car'; 
	return;

	function employeeId() {
		console.log(product);
	}
}
foo();
  1. undefined
  2. Type Error
  3. 'Car'
  4. ReferenceError: product is not defined

Answer: 1) undefined

Q. What would be the output of following code?

(function foo() {
	bar();

	function bar() {
		abc();
		console.log(typeof abc);
	}

	function abc() {
		console.log(typeof bar);
	}
}());
  1. undefined undefined
  2. Type Error
  3. function function
  4. ReferenceError: bar is not defined

Answer: 3) function function

Q. What would be the output of following code?

(function() {
	'use strict';

	var person = {
		name: 'John'
	};
	person.salary = '10000$';
	person['country'] = 'USA';

	Object.defineProperty(person, 'phoneNo', {
		value: '8888888888',
		enumerable: true
	})

	console.log(Object.keys(person)); 
})();
  1. Type Error
  2. undefined
  3. ["name", "salary", "country", "phoneNo"]
  4. ["name", "salary", "country"]

Answer: 3) ["name", "salary", "country", "phoneNo"]

Q. What would be the output of following code?

(function() {
	'use strict';

	var person = {
		name: 'John'
	};
	person.salary = '10000$';
	person['country'] = 'USA';

	Object.defineProperty(person, 'phoneNo', {
		value: '8888888888',
		enumerable: false
	})

	console.log(Object.keys(person)); 
})();
  1. Type Error
  2. undefined
  3. ["name", "salary", "country", "phoneNo"]
  4. ["name", "salary", "country"]

Answer: 4) ["name", "salary", "country"]

Q. What would be the output of following code?

(function() {
	var objA = {
		foo: 'foo',
		bar: 'bar'
	};
	var objB = {
		foo: 'foo',
		bar: 'bar'
	};
	console.log(objA == objB);
	console.log(objA === objB);
}());
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code ?

(function() {
	var objA = new Object({foo: "foo"});
	var objB = new Object({foo: "foo"});
	console.log(objA == objB);
	console.log(objA === objB);
}());
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code ?

(function() {
	var objA = Object.create({
		foo: 'foo'
	});
	var objB = Object.create({
		foo: 'foo'
	});
	console.log(objA == objB);
	console.log(objA === objB);
}());
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code ?

(function() {
	var objA = Object.create({
		foo: 'foo'
	});
	var objB = Object.create(objA);
	console.log(objA == objB);
	console.log(objA === objB);
}());
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 2) false false

Q. What would be the output of following code ?

(function() {
	var objA = Object.create({
		foo: 'foo'
	});
	var objB = Object.create(objA);
	console.log(objA.toString() == objB.toString());
	console.log(objA.toString() === objB.toString());
}());
  1. false true
  2. false false
  3. true false
  4. true true

Answer: 4) true true

Q. What would be the output of following code ?

(function() {
	var objA = Object.create({
		foo: 'foo'
	});
	var objB = objA;
	console.log(objA == objB);
	console.log(objA === objB);
	console.log(objA.toString() == objB.toString());
	console.log(objA.toString() === objB.toString());
}());
  1. true true true false
  2. true false true true
  3. true true true true
  4. true true false false

Answer: 3) true true true true

Q. What would be the output of following code ?

(function() {
	var objA = Object.create({
		foo: 'foo'
	});
	var objB = objA;
	objB.foo = 'bar';
	console.log(objA.foo);
	console.log(objB.foo);
}());
  1. foo bar
  2. bar bar
  3. foo foo
  4. bar foo

Answer: 2) bar bar

Q. What would be the output of following code ?

(function() {
	var objA = Object.create({
		foo: 'foo'
	});
	var objB = objA;
	objB.foo = 'bar';

	delete objA.foo;
	console.log(objA.foo);
	console.log(objB.foo);
}());
  1. foo bar
  2. bar bar
  3. foo foo
  4. bar foo

Answer: 3) foo foo

Q. What would be the output of following code ?

(function() {
	var objA = {
		foo: 'foo'
	};
	var objB = objA;
	objB.foo = 'bar';

	delete objA.foo;
	console.log(objA.foo);
	console.log(objB.foo);
}());
  1. foo bar
  2. undefined undefined
  3. foo foo
  4. undefined bar

Answer: 2) undefined undefined

Q. What would be the output of following code?

(function() {
	var array = new Array('100');
	console.log(array);
	console.log(array.length);
}());
  1. undefined undefined
  2. [undefined × 100] 100
  3. ["100"] 1
  4. ReferenceError: array is not defined

Answer: 3) ["100"] 1

Q. What would be the output of following code?

(function() {
	var array1 = [];
	var array2 = new Array(100);
	var array3 = new Array(['1',2,'3',4,5.6]);
	console.log(array1);
	console.log(array2);
	console.log(array3);
	console.log(array3.length);
}());
  1. [] [] [Array[5]] 1
  2. [] [undefined × 100] Array[5] 1
  3. [] [] ['1',2,'3',4,5.6] 5
  4. [] [] [Array[5]] 5

Answer: 1) [] [] [Array[5]] 1

Q. What would be the output of following code?

(function () {
  var array = new Array('a', 'b', 'c', 'd', 'e');
  array[10] = 'f';
  delete array[10];
  console.log(array.length);
}());
  1. 11
  2. 5
  3. 6
  4. undefined

Answer: 1) 11

Q. What would be the output of following code?

(function(){
	var animal = ['cow','horse'];
		animal.push('cat');
		animal.push('dog','rat','goat');
		console.log(animal.length);
})();
  1. 4
  2. 5
  3. 6
  4. undefined

Answer: 3) 6

Q. What would be the output of following code?

(function(){
	var animal = ['cow','horse'];
		animal.push('cat');
		animal.unshift('dog','rat','goat');
		console.log(animal);
})();
  1. [ 'dog', 'rat', 'goat', 'cow', 'horse', 'cat' ]
  2. [ 'cow', 'horse', 'cat', 'dog', 'rat', 'goat' ]
  3. Type Error
  4. undefined

Answer: 1) [ 'dog', 'rat', 'goat', 'cow', 'horse', 'cat' ]

Q. What would be the output of following code?

(function(){
	var array = [1,2,3,4,5];
	console.log(array.indexOf(2));
	console.log([{name: 'John'},{name : 'John'}].indexOf({name:'John'}));
	console.log([[1],[2],[3],[4]].indexOf([3]));
	console.log("abcdefgh".indexOf('e'));
})();
  1. 1 -1 -1 4
  2. 1 0 -1 4
  3. 1 -1 -1 -1
  4. 1 undefined -1 4

Answer: 1) 1 -1 -1 4

Q. What would be the output of following code?

(function(){
	var array = [1,2,3,4,5,1,2,3,4,5,6];
	console.log(array.indexOf(2));
	console.log(array.indexOf(2,3));
	console.log(array.indexOf(2,10));
})();
  1. 1 -1 -1
  2. 1 6 -1
  3. 1 1 -1
  4. 1 undefined undefined

Answer: 2) 1 6 -1

Q. What would be the output of following code?

(function(){
	var numbers = [2,3,4,8,9,11,13,12,16];
	var even = numbers.filter(function(element, index){
		return element % 2 === 0; 
	});
	console.log(even);

	var containsDivisibleby3 = numbers.some(function(element, index){
		return element % 3 === 0;
	});

	console.log(containsDivisibleby3);	
})();
  1. [ 2, 4, 8, 12, 16 ] [ 0, 3, 0, 0, 9, 0, 12]
  2. [ 2, 4, 8, 12, 16 ] [ 3, 9, 12]
  3. [ 2, 4, 8, 12, 16 ] true
  4. [ 2, 4, 8, 12, 16 ] false

Answer: 3) [ 2, 4, 8, 12, 16 ] true

Q. What would be the output of following code?

(function(){
	var containers = [2,0,false,"", '12', true];
	var containers = containers.filter(Boolean);
	console.log(containers);
	var containers = containers.filter(Number);
	console.log(containers);
	var containers = containers.filter(String);
	console.log(containers);
	var containers = containers.filter(Object);
	console.log(containers);		
})();
  1. [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ]
  2. [false, true] [ 2 ] ['12'] [ ]
  3. [2,0,false,"", '12', true] [2,0,false,"", '12', true] [2,0,false,"", '12', true] [2,0,false,"", '12', true]
  4. [ 2, '12', true ] [ 2, '12', true, false ] [ 2, '12', true,false ] [ 2, '12', true,false]

Answer: 1) [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ]

Q. What would be the output of following code?

(function(){
	var list = ['foo','bar','john','ritz'];
	    console.log(list.slice(1));	
	    console.log(list.slice(1,3));
	    console.log(list.slice());
	    console.log(list.slice(2,2));
	    console.log(list);				
})();
  1. [ 'bar', 'john', 'ritz' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
  2. [ 'bar', 'john', 'ritz' ] [ 'bar', 'john','ritz ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
  3. [ 'john', 'ritz' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
  4. [ 'foo' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]

Answer: 1) [ 'bar', 'john', 'ritz' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]

Q. What would be the output of following code?

(function(){
	var list = ['foo','bar','john'];
	    console.log(list.splice(1));		
	    console.log(list.splice(1,2));
	    console.log(list);			
})();
  1. [ 'bar', 'john' ] [] [ 'foo' ]
  2. [ 'bar', 'john' ] [] [ 'bar', 'john' ]
  3. [ 'bar', 'john' ] [ 'bar', 'john' ] [ 'bar', 'john' ]
  4. [ 'bar', 'john' ] [] []

Answer: 1. [ 'bar', 'john' ] [] [ 'foo' ]

Q. What would be the output of following code?

(function(){
	var arrayNumb = [2, 8, 15, 16, 23, 42];
	arrayNumb.sort();
	console.log(arrayNumb);
})();
  1. [2, 8, 15, 16, 23, 42]
  2. [42, 23, 26, 15, 8, 2]
  3. [ 15, 16, 2, 23, 42, 8 ]
  4. [ 2, 8, 15, 16, 23, 42 ]

Answer: 3. [ 15, 16, 2, 23, 42, 8 ]

Q. What would be the output of following code?

function funcA(){
	console.log("funcA ", this);
	(function innerFuncA1(){
		console.log("innerFunc1", this);
		(function innerFunA11(){
			console.log("innerFunA11", this);
		})();
	})();
}
	
console.log(funcA());
  1. funcA Window {...} innerFunc1 Window {...} innerFunA11 Window {...}
  2. undefined
  3. Type Error
  4. ReferenceError: this is not defined

Answer: 1)

Q. What would be the output of following code?

var obj = {
	message: "Hello",
	innerMessage: !(function() {
		console.log(this.message);
	})()
};
	
console.log(obj.innerMessage);
  1. ReferenceError: this.message is not defined
  2. undefined
  3. Type Error
  4. undefined true

Answer: 4) undefined true

Q. What would be the output of following code?

var obj = {
	message: "Hello",
	innerMessage: function() {
		return this.message;
	}
};
	
console.log(obj.innerMessage());
  1. Hello
  2. undefined
  3. Type Error
  4. ReferenceError: this.message is not defined

Answer: 1) Hello

Q. What would be the output of following code?

var obj = {
  message: 'Hello',
  innerMessage: function () {
    (function () {
      console.log(this.message);
    }());
  }
};
console.log(obj.innerMessage());
  1. Type Error
  2. Hello
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 3) undefined

Q. What would be the output of following code?

var obj = {
  message: 'Hello',
  innerMessage: function () {
  	var self = this;
    (function () {
      console.log(self.message);
    }());
  }
};
console.log(obj.innerMessage());
  1. Type Error
  2. 'Hello'
  3. undefined
  4. ReferenceError: self.message is not defined

Answer: 2) 'Hello'

Q. What would be the output of following code?

function myFunc(){
	console.log(this.message);
}
myFunc.message = "Hi John";
	
console.log(myFunc());
  1. Type Error
  2. 'Hi John'
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 3) undefined

Q. What would be the output of following code?

function myFunc(){
	console.log(myFunc.message);
}
myFunc.message = "Hi John";
	
console.log(myFunc());
  1. Type Error
  2. 'Hi John'
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 2) 'Hi John'

Q. What would be the output of following code?

function myFunc() {
  myFunc.message = 'Hi John';
  console.log(myFunc.message);
}
console.log(myFunc());
  1. Type Error
  2. 'Hi John'
  3. undefined
  4. ReferenceError: this.message is not defined

Answer: 2) 'Hi John'

Q. What would be the output of following code?

function myFunc(param1,param2) {
  console.log(myFunc.length);
}
console.log(myFunc());
console.log(myFunc("a","b"));
console.log(myFunc("a","b","c","d"));
  1. 2 2 2
  2. 0 2 4
  3. undefined
  4. ReferenceError

Answer: a) 2 2 2

Q. What would be the output of following code?

function myFunc() {
  console.log(arguments.length);
}
console.log(myFunc());
console.log(myFunc("a","b"));
console.log(myFunc("a","b","c","d"));
  1. 2 2 2
  2. 0 2 4
  3. undefined
  4. ReferenceError

Answer: 2) 0 2 4

Q. What would be the output of following code?

function Person(name, age){
	this.name = name || "John";
	this.age = age || 24;
	this.displayName = function(){
		console.log(this.name);
	}
}

Person.name = "John";
Person.displayName = function(){
	console.log(this.name);
}

var person1 = new Person('John');
	person1.displayName();
	Person.displayName();
  1. John Person
  2. John John
  3. John undefined
  4. John John

Answer: 1) John Person

Q. What would be the output of following code?

function passWordMngr() {
	var password = '12345678';
	this.userName = 'John';
	return {
		pwd: password
	};
}
// Block End
var userInfo = passWordMngr();
console.log(userInfo.pwd);
console.log(userInfo.userName);
  1. 12345678 Window
  2. 12345678 John
  3. 12345678 undefined
  4. undefined undefined

Answer: 3) 12345678 undefined

Q. What would be the output of following code?

var employeeId = 'aq123';
function Employee() {
  this.employeeId = 'bq1uy';
}
console.log(Employee.employeeId);
  1. Reference Error
  2. aq123
  3. bq1uy
  4. undefined

Answer: 4) undefined

Q. What would be the output of following code?

var employeeId = 'aq123';

function Employee() {
	this.employeeId = 'bq1uy';
}
console.log(new Employee().employeeId);
Employee.prototype.employeeId = 'kj182';
Employee.prototype.JobId = '1BJKSJ';
console.log(new Employee().JobId);
console.log(new Employee().employeeId);
  1. bq1uy 1BJKSJ bq1uy undefined
  2. bq1uy 1BJKSJ bq1uy
  3. bq1uy 1BJKSJ kj182
  4. undefined 1BJKSJ kj182

Answer: 2) bq1uy 1BJKSJ bq1uy

Q. What would be the output of following code?

var employeeId = 'aq123';
(function Employee() {
	try {
		throw 'foo123';
	} catch (employeeId) {
		console.log(employeeId);
	}
	console.log(employeeId);
}());
  1. foo123 aq123
  2. foo123 foo123
  3. aq123 aq123
  4. foo123 undefined

Answer: 1) foo123 aq123

Q. What would be the output of following code?

(function() {
	var greet = 'Hello World';
	var toGreet = [].filter.call(greet, function(element, index) {
		return index > 5;
	});
	console.log(toGreet);
}());
  1. Hello World
  2. undefined
  3. World
  4. [ 'W', 'o', 'r', 'l', 'd' ]

Answer: 4) [ 'W', 'o', 'r', 'l', 'd' ]

Q. What would be the output of following code?

(function() {
	var fooAccount = {
		name: 'John',
		amount: 4000,
		deductAmount: function(amount) {
			this.amount -= amount;
			return 'Total amount left in account: ' + this.amount;
		}
	};
	var barAccount = {
		name: 'John',
		amount: 6000
	};
	var withdrawAmountBy = function(totalAmount) {
		return fooAccount.deductAmount.bind(barAccount, totalAmount);
	};
	console.log(withdrawAmountBy(400)());
	console.log(withdrawAmountBy(300)());
}());
  1. Total amount left in account: 5600 Total amount left in account: 5300
  2. undefined undefined
  3. Total amount left in account: 3600 Total amount left in account: 3300
  4. Total amount left in account: 5600 Total amount left in account: 5600

Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300

Q. What would be the output of following code?

(function() {
	var fooAccount = {
		name: 'John',
		amount: 4000,
		deductAmount: function(amount) {
			this.amount -= amount;
			return this.amount;
		}
	};
	var barAccount = {
		name: 'John',
		amount: 6000
	};
	var withdrawAmountBy = function(totalAmount) {
		return fooAccount.deductAmount.apply(barAccount, [totalAmount]);
	};
	console.log(withdrawAmountBy(400));
	console.log(withdrawAmountBy(300));
	console.log(withdrawAmountBy(200));
}());
  1. 5600 5300 5100
  2. 3600 3300 3100
  3. 5600 3300 5100
  4. undefined undefined undefined

Answer: 1) 5600 5300 5100

Q. What would be the output of following code?

(function() {
	var fooAccount = {
		name: 'John',
		amount: 6000,
		deductAmount: function(amount) {
			this.amount -= amount;
			return this.amount;
		}
	};
	var barAccount = {
		name: 'John',
		amount: 4000
	};
	var withdrawAmountBy = function(totalAmount) {
		return fooAccount.deductAmount.call(barAccount, totalAmount);
	};
	console.log(withdrawAmountBy(400));
	console.log(withdrawAmountBy(300));
	console.log(withdrawAmountBy(200));
}());
  1. 5600 5300 5100
  2. 3600 3300 3100
  3. 5600 3300 5100
  4. undefined undefined undefined

Answer: 2) 3600 3300 3100

Q. What would be the output of following code?

(function greetNewCustomer() {
	console.log('Hello ' + this.name);
}.bind({
	name: 'John'
})());
  1. Hello John
  2. Reference Error
  3. Window
  4. undefined

Answer: 1) Hello John

Q. What would be the output of following code ?

function getDataFromServer(apiUrl){
    var name = "John";
	return {
		then : function(fn){
			fn(name);
		}
	}
}

getDataFromServer('www.google.com').then(function(name){
	console.log(name);
});
  1. John
  2. undefined
  3. Reference Error
  4. fn is not defined

Answer: 1) John

Q. What would be the output of following code?

(function(){
	var arrayNumb = [2, 8, 15, 16, 23, 42];
	Array.prototype.sort = function(a,b){
		return a - b;
	};
	arrayNumb.sort();
	console.log(arrayNumb);
})();

(function(){
	var numberArray = [2, 8, 15, 16, 23, 42];
	numberArray.sort(function(a,b){
		if(a == b){
			return 0;
		}else{
			return a < b ? -1 : 1;
		}
	});
	console.log(numberArray);
})();

(function(){
	var numberArray = [2, 8, 15, 16, 23, 42];
	numberArray.sort(function(a,b){
		return a-b;
	});
	console.log(numberArray);
})();
  1. [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]
  2. undefined undefined undefined
  3. [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2]
  4. Reference Error

Answer: 1) [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]

Q. What would be the output of following code?

(function(){
	function sayHello(){
		var name = "Hi John";
		return 
		{
			fullName: name
		}
	}
	console.log(sayHello().fullName);
})();
  1. Hi John
  2. undefined
  3. Reference Error
  4. Uncaught TypeError: Cannot read property 'fullName' of undefined

Answer: 4) Uncaught TypeError: Cannot read property 'fullName' of undefined

Q. What would be the output of following code?

function getNumber(){
	return (2,4,5);
}

var numb = getNumber();
console.log(numb);
  1. 5
  2. undefined
  3. 2
  4. (2,4,5)

Answer: 1) 5

Q. What would be the output of following code?

function getNumber(){
	return;
}

var numb = getNumber();
console.log(numb);
  1. null
  2. undefined
  3. ""
  4. 0

Answer: 2) undefined

Q. What would be the output of following code?

function mul(x){
	return function(y){
		return [x*y, function(z){
			return x*y + z;
		}];
	}
}

console.log(mul(2)(3)[0]);
console.log(mul(2)(3)[1](4));
  1. 6, 10
  2. undefined undefined
  3. Reference Error
  4. 10, 6

Answer: 1) 6, 10

Q. What would be the output of following code?

function mul(x) {
	return function(y) {
		return {
			result: x * y,
			sum: function(z) {
				return x * y + z;
			}
		};
	};
}
console.log(mul(2)(3).result);
console.log(mul(2)(3).sum(4));
  1. 6, 10
  2. undefined undefined
  3. Reference Error
  4. 10, 6

Answer: 1) 6, 10

Q. What would be the output of following code?

function mul(x) {
	return function(y) {
		return function(z) {
			return function(w) {
				return function(p) {
					return x * y * z * w * p;
				};
			};
		};
	};
}
console.log(mul(2)(3)(4)(5)(6));
  1. 720
  2. undefined
  3. Reference Error
  4. Type Error

Answer: 1) 720

Q. What is the value of foo?

var foo = 10 + '20';

Answer: '1020', because of type coercion from Number to String

Q. How would you make this work?

add(2, 5); // 7
add(2)(5); // 7

Answer: A general solution for any number of parameters

'use strict';

let sum = (arr) => arr.reduce((a, b) => a + b);
let addGenerator = (numArgs, prevArgs) => {
  return function () {
    let totalArgs = prevArgs.concat(Array.from(arguments));
    if (totalArgs.length === numArgs) {
      return sum(totalArgs);
    }
    return addGenerator(numArgs, totalArgs);
  };
};

let add = addGenerator(2, []);

add(2, 5); // 7
add(2)(5); // 7
add()(2, 5); // 7
add()(2)(5); // 7
add()()(2)(5); // 7

Q. What value is returned from the following statement?

"i'm a lasagna hog".split("").reverse().join("");

Answer: It's actually a reverse method for a string - 'goh angasal a m\'i'

Q. What is the value of window.foo?

( window.foo || ( window.foo = "bar" ) );

Answer: Always 'bar'

Q. What is the outcome of the two alerts below?

var foo = "Hello";
(function() {
  var bar = " World";
  alert(foo + bar);
})();
alert(foo + bar);

Answer:

  • First: Hello World
  • Second: Throws an exception, ReferenceError: bar is not defined

Q. What is the value of foo.length?

var foo = [];
foo.push(1);
foo.push(2);

Answer: .push is mutable - 2

Q. What is the value of foo.x?

var foo = {n: 1};
var bar = foo;
foo.x = foo = {n: 2};

Answer: undefined. Rather, bar.x is {n: 2}.

foo.x = foo = {n: 2} is the same as foo.x = (foo = {n: 2}). It is because a left term is first referenced and then a right term is evaluated when an assignment is performed in JavaScript. When foo.x is referenced, it refers to an original object, {n: 1}. So, when the result of the right term, {n: 2}, is evaluated, it will assigned to the original object, which is at the moment referenced by bar.

Q. What does the following code print?*

console.log('one');
setTimeout(function() {
  console.log('two');
}, 0);
console.log('three');

Answer: one, three and two. It's because console.log('two'); will be invoked in the next event loop.

Q. What would be the result of 1+2+'3'?

The output is going to be 33. Since 1 and 2 are numeric values, the result of first two digits is going to be a numeric value 3. The next digit is a string type value because of that the addition of numeric value 3 and string type value 3 is just going to be a concatenation value 33.

Q. Write a script that returns the number of occurrences of character given a string as input?

function countCharacters(str) {
  return str.replace(/ /g, '').toLowerCase().split('').reduce((arr, character) => {
    if (character in arr) {
      arr[character]++;
    } else {
      arr[character] = 1;
    }
    return arr;
  }, {});
}
console.log(countCharacters("the brown fox jumps over the lazy dog"));

Q. What is the value of foo?

var foo = 10 + '20';

Answer: '1020', because of type coercion from Number to String

Q. How would you make this work?

add(2, 5); // 7
add(2)(5); // 7

Answer: A general solution for any number of parameters

'use strict';

let sum = (arr) => arr.reduce((a, b) => a + b);
let addGenerator = (numArgs, prevArgs) => {
  return function () {
    let totalArgs = prevArgs.concat(Array.from(arguments));
    if (totalArgs.length === numArgs) {
      return sum(totalArgs);
    }
    return addGenerator(numArgs, totalArgs);
  };
};

let add = addGenerator(2, []);

add(2, 5); // 7
add(2)(5); // 7
add()(2, 5); // 7
add()(2)(5); // 7
add()()(2)(5); // 7

Q. What value is returned from the following statement?

"i'm a lasagna hog".split("").reverse().join("");

Answer: It's actually a reverse method for a string - 'goh angasal a m\'i'

Q. What is the value of window.foo?

( window.foo || ( window.foo = "bar" ) );

Answer: Always 'bar'

Q. What is the outcome of the two alerts below?

var foo = "Hello";
(function() {
  var bar = " World";
  alert(foo + bar);
})();
alert(foo + bar);

Answer:

  • First: Hello World
  • Second: Throws an exception, ReferenceError: bar is not defined

Q. What is the value of foo.length?

var foo = [];
foo.push(1);
foo.push(2);

Answer: .push is mutable - 2

Q. What is the value of foo.x?

var foo = {n: 1};
var bar = foo;
foo.x = foo = {n: 2};

Answer: undefined. Rather, bar.x is {n: 2}.

foo.x = foo = {n: 2} is the same as foo.x = (foo = {n: 2}). It is because a left term is first referenced and then a right term is evaluated when an assignment is performed in JavaScript. When foo.x is referenced, it refers to an original object, {n: 1}. So, when the result of the right term, {n: 2}, is evaluated, it will assigned to the original object, which is at the moment referenced by bar.

Q. What does the following code print?

console.log('one');
setTimeout(function() {
  console.log('two');
}, 0);
console.log('three');

Answer: one, three and two. It's because console.log('two'); will be invoked in the next event loop.

Q. For which value of x the results of the following statements are not the same?

//  if( x <= 100 ) {...}
if( !(x > 100) ) {...}

Answer: NaN <= 100 is false and NaN > 100 is also false, so if the value of x is NaN, the statements are not the same.

The same holds true for any value of x that being converted to Number, returns NaN, e.g.: undefined, [1,2,5], {a:22}, etc.

Q. what is g value?

f = g = 0;
(function () {
  try {
    f = function() {
      return f();
    };
    f();
  } catch (e) {
    return g++ && f();
  } finally {
    return ++g;
  }
  function f() { g += 5; return 0; }
}) ();

Q. what is g value?

f = g = 0;
(function () {
  try {
    f = function() {
      return f();
    } && f();
  } catch (e) {
    return g++ && f();
  } finally {
    return ++g;
  }
  function f() { g += 5; return 0; }
}) ();

Q. what will be the output?

function b(b) {
  return this.b && b(b)
}
b(b.bind(b))

Q. what will be the output?

c = (c) => {
  return this.c && c(c)
}
c(c.bind(c))

Q. Predict the output of the following JavaScript code?

var g = 0;
g = 1 && g++;
console.log(g);

Q. Predict the output of the following JavaScript code?

!function(){}()
function(){}()
true && function(){}()
(function(){})()
function(){}
!function(){}

Q. what will expression return?

var a = b = true, c = (a) => a;
(function a(a = c(b).a = c = () => a) { return a(); })()

Q. Predict the output of the following JavaScript code?

var a = true;
(a = function () { return a })()

Q. what will be the output?

var v = 0;
try {
  throw v = (function(c) { throw v = function(a){ return v; } })();
} catch (e) {
  console.log (e()());
}