JavaScript

Question 1

Accessing array elements

Answer 1

array[index]

Example:

var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37];
primes[0]; // 2
primes[3]; // 7
primes[150]; // undefined

Question 2

Array literals

Answer 2

var arrayName = [element0, element1, …, elementN]

Example:

var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37];

var junk = [10, “Hello”, Math, console, { lots: “of different types” }];

Question 3

Multi-dimensional arrays

Answer 3

var multidimensionalArray = [[1,2,3],[4,5,6],[7,8,9]]
 // two dimensions, 3x3

Question 4

Array constructor

Answer 4

Example 1:

var myArray = new Array(45 , "Hello World!" , true , 3.2 , undefined);
 console.log(myArray);

Example 2:

var stuff = new Array();

stuff[0] = 34;
stuff[4] = 20;

stuff // [34, undefined, undefined, undefined, 20]

Question 5

Accessing nested array elements

Answer 5

array[index][index]….

Example:

var myMultiArray = [
[1,2,3,4,5, [1,2,3,4,5] ],
[6,7,8,9,10 , [1,2,3,4,6] ],
[11,12,13,14,15 , [1,2,3,4,5] ],
[16,17,18,19,20, [1,2,3,4,5] ]
];

console.log( myMultiArray[1][5][4] ); //Outputs 6 , the value in the last element of the last element of the second element of myMultiArray.

Question 6

Boolean literals

Answer 6

true
false

Question 7

Boolean logical operators

Answer 7

if ( true && false )alert("Not executed!");
 //because the second expression is false

if( false || true )alert("Executed!");
 //because any one of the expression is true

if( !false )alert("Executed!");
 // because !false evaluates to true

!!true // remains true

Question 8

Operator Precedance

Answer 8

// Brackets - have the highest precedence
 // ! - lower than Brackets
 // && - lower than !
 // || - the lowest

if(true && !!false || true)alert(“Guess again ??”);

/* Executed , here is the evaluation process-
true && !!false || true - becomes
true && false || true - (no brackets present , so ! evaluated ) becomes
false || true - (then && evaluated) which becomes true */

Question 9

Operator Associativity

Answer 9

Determines the order in which operators of the same precedence are processed. For example, consider an expression: a * b * c . Left-associativity (left-to-right) means that it is processed as (a * b) * c, while right-associativity (right-to-left) means it is interpreted as a * (b * c). */

// Brackets , && , || have left to right associativity
 // ! has right to left associativity
 // So ,

!false && !!false //false
 // evaluated in the manner - !false && false - true && false - false

Question 10

Comparison operator

Answer 10

x === y // returns true if two things are equal
x !== y // returns true if two things are not equal
x <= y // returns true if x is less than or equal to y
x >= y // returns true if x is greater than or equal to y
x < y // returns true if x is less than y
x > y // returns true if x is greater than y

Question 11

Other ways of deriving true and false

Answer 11

if(1)console.log(“True!”); // output True! , since any non-zero number is considered to be true

if(0)console.log(“I doubt if this gets executed”); // not executed , since 0 is considered to be false

if(“Hello”)alert(“So, any non-empty String is also true.”); //Gets executed

if(““)alert(“Hence , an empty String is false”); // Not executed

Question 12

Difference between == and ===

Answer 12

== does just value checking ( no type checking ) , whereas , === does both value checking and type checking . It is always advisable that you never use == , because == often produces unwanted results

Example:

‘1’ == 1 //true (same value)
‘1’ === 1 // false (not the same type)

true == 1 // true (because 1 stands for true ,though it’s not the same type)
true === 1 // false (not the same type)

Question 13

Comments

Answer 13

// Single line

/*
Multi
line
*/

Question 14

console.log

Answer 14

Prints to the console, good for debugging.

Example:
console.log(‘Poker night!’);

Question 15

console.time

Answer 15

console. time(timerName);
console. timeEnd(timerName);

Example:

console.time(“My Math”);
var x = 5 + 5;
console.log(x);
console.timeEnd(“My Math”);
console.log(“Done the math.”);
/* Output:
10
My Math: (time taken)
Done the math.

Question 16

function

Answer 16

function name(argument1 , argument2 …. argumentN){
statement1;
statement2;
statementN;
}

Example:

function greet(name) {
   return "Hello" + name + "!";
 }

Question 17

function calling

Answer 17

functionName(argument1, argument2, …, argumentN);

Question 18

function hoisting

Answer 18

hoistedFunction(); // Hello! I am defined immediately!
notHoistedFunction(); // ReferenceError: notHoistedFunction is not defined

function hoistedFunction () {
   console.log('Hello! I am defined immediately!');
 }

var notHoistedFunction = function () {
   console.log('I am not defined immediately.');
 }

Question 19

if

Answer 19

if (condition) {
   // code that runs if the condition is true
 }

Question 20

else

Answer 20

if (condition) {
   // statement1: code that runs if condition is true
 } else {
   // statement2: code that runs if condition is false
 }

Question 21

else if

Answer 21

if (condition1) {
statement1;
} else if (condition2) {
statement2;
} else {
statement3;
}

Question 22

for loops

Answer 22

for ([var i = startValue];[i \< endValue]; [i+=stepValue]) {
     // Your code here
 }

Example:

for (var i = 0; i \< 5; i++) {
   console.log(i); // Prints the numbers from 0 to 4
 }

var i; // “outsourcing” the definition
for (i = 10; i >= 1; i–) {
console.log(i); // Prints the numbers from 10 to 1
}

Question 23

for loops: all three statements are ____

Answer 23

/* Note that all of the three statements are optional, i.e. , */
var i = 9;
for(;;){
if(i === 0)break;
console.log(i);
i–;
}

//This loop is perfectly valid.

Question 24

while loops

Answer 24

var x = 0;
 while (x \< 5) {
   console.log(x); // Prints numbers from 0 to 4
   x++;
 }

var x = 10;
while (x <= 5) {
console.log(x); // Won’t be executed
x++;
}

Question 25

do while

Answer 25

do {
   // Your code here
 } while (condition);

Example:

var x = 10;
 do {
     console.log(x); // Prints 10
     x++;
 } while (x \<= 5);

Question 26

random

Answer 26

Math.random(); // A random number between 0 and 1.

Question 27

floor

Answer 27

Math.floor(expression)

Returns the largest integer less than or equal to a number.

Math.floor(9.99); // 9

Math.floor(1 + 0.5); // 1
Math.floor(Math.random() * X + 1); // Returns a random number between 1 and X

Question 28

Returns base raised to exponent.

Answer 28

Math.pow(base,exponent)

Question 29

ceil

Answer 29

Math.ceil(expression)

Returns the smallest integer greater than or equal to a number.

Math.ceil(45.4); // 46
Math.ceil(4 - 1.9); // 3

Question 30

Returns the ratio of the circumference of a circle to its diameter

Answer 30

Math.PI

Example:

Math.round(Math.PI); // rounds the value of PI ,gives 3
Math.ceil(Math.PI); // 4

Question 31

Returns the square root of a number.

Answer 31

Math.sqrt(expression)

Question 32

Modulus

Answer 32

Returns the remainder left after dividing the left hand side with the right hand side.

number1 % number2

Example:

14 % 9 // returns 5

Question 33

isNaN

Answer 33

isNaN([value])

Returns true if the given number is not a number , else returns false.

var user_input = prompt(“Enter a number”); // Enter “a number”

if(isNaN(user_input))
alert(“I told you to enter a number.”);

//alert executed , since “a number” is not a number

//Another important thing:

if( isNaN(“3”) )
alert(“bad”);

//Not executed , because the string “3” gets converted into 3 ,and 3 is a number

Question 34

Prefix/Postfix Increment/Decrement

Answer 34

++variable
–variable
variable++
variable–

var x = 15; // x has a value of 15
var y = x++;
// since it is postfix , the value of x (15) is first assigned to y and then the value of x is incremented by 1
console.log(y); //15
console.log(x); //16

var a = 15; // a has a value of 15
var b = ++a;
// since it is prefix , the value of a (15) is first incremented by 1 and then the value of x is assigned to b
console.log(b); //16
console.log(a); //16

Question 35

Object Literals

Answer 35

{
“property 1”: value1,
property2: value2,
number: value3
}

Example:

var obj = {
   name: "Bob",
   married: true,
   "mother's name": "Alice",
   "year of birth": 1987,
   getAge: function () {
     return 2012 - obj["year of birth"];
   },
   1: 'one'
 };

Question 36

Property Access

Answer 36

name1[string]
name2.identifier

Example:

obj[‘name’]; // ‘Bob’

obj. name; // ‘Bob’
obj. getAge(); // 24

Question 37

Classes

Answer 37

A class can be thought of as a template to create many objects with similar qualities.

SubClass.prototype = new SuperClass();

var Lieutenant = function (age) {

this.rank = “Lieutenant”;
this.age = age;
};

Lieutenant.prototype = new PoliceOfficer();

Lieutenant.prototype.getRank = function () {
return this.rank;
};

var John = new Lieutenant(67);

John.getJob(); // ‘Police Officer’
John.getRank(); // ‘Lieutenant’
John.retire(); // true

Question 38

confirm

Answer 38

confirm(“message”) //returns true if confirmed, false otherwise

Example:

if ( confirm(“Are you sure you want to delete this post?”) ) {
deletePost();
}

Question 39

prompt

Answer 39

prompt(message,value)
prompt(message,value) // value is a string containing the default value displayed in the text input field.

Displays a dialog with an optional message prompting the user to input some text. If the user clicks the “Cancel” button , null is returned.

Question 40

Changes the cases of all the alphabetical letters in the string.

Answer 40

string. toUpperCase()
string. toLowerCase()

Question 41

Removes whitespace from both ends of the string.

Answer 41

string.trim();

Example:

” a “.trim(); // ‘a’
“ a a “.trim(); // ‘a a’

Question 42

Returns a string with the first match substring replaced with a new substring

Answer 42

.replace();

“original string”.replace(“original”, “replaced”); // returns “replaced string”

Question 43

Returns the specified character from a string.

Answer 43

.charAt()

string.charAt(index) // index is an integer between 0 and 1 less than the length of the string.

“Hello World!”.charAt(0); // ‘H’
“Hello World!”.charAt(234); // ‘’