JavaScript

Question 1

Name the primitive types of javascript

Answer 1

1. number 2. string 3. boolean 4. null 5. undefined

Question 2

Describe lexical scoping

Answer 2

Lexical scoping, aka static scoping, refers to the process of determining a variable’s SCOPE based on its position within the body of code. For example, variables declared outside of a function have global scope, whereas variables declared inside a function have local scope (visible to members of the function only)

Question 3

What is a lambda language

Answer 3

In simple terms, a lambda language is one that allows functions to be passed to other functions, with the passed function being treated like any other variable

Question 4

In JavaScript, 1 and 1.0 are the same value because _

Answer 4

1 and 1.0 are the same value because JavaScript has no separate integer type, it has a single number type

Question 5

Describe NaN

Answer 5

NaN is a number value that is the result of an operation that cannot produce a normal result. e.g. multiplying a number with a string

Question 6

How can detect NaN

Answer 6

You can detect NaN by using the isNaN function, e.g. isNan( number)

Question 7

In JavaScript, all characters are _ bit wide

Answer 7

16

Question 8

In JavaScript, all numbers are _ bit floating-point

Answer 8

64

Question 9

Strings are immutable. Describe

Answer 9

A string cannot be changed once its made. It is, however, easy to create a new string by concatenating other strings together using the + operator

Question 10

What is a statement

Answer 10

A statement is a sentence or command, which ends with a semicolon. Statements make something happen, e.g. var name; is a statement that declares a variable called name

Question 11

True or false, blocks in JavaScript do not create a new scope

Answer 11

True, blocks in JavaScript do not create a new scope, so variables should be defined at the top of a function, not in blocks

Question 12

What are the falsy values?

Answer 12

1. false 2. null 3. undefined 4. empty string ‘’ 5. number 0 6. NaN

Question 13

Explain the switch statement

Answer 13

The switch statement performs a multiway branch by comparing the expression against cases for equality switch (expression){ case expression1 : (case can contain one or more expressions) //code to be executed break; case expression2: //code to be executed break; default: //code to be executed }

Question 14

What is an expression

Answer 14

An expression is a phrase that can be evaluated into a value by the JavaScript interpreter. For example var name = “Succeed”

Question 15

What is the global object

Answer 15

Its a common namespace where all top-level variables for all compilation units are stored

Question 16

What are the advantages of JavaScript having a single number type?

Answer 16

1. Problems of overflow in short integers are avoided 2. A large class of numeric type errors is avoided

Question 17

When is it appropriate to use a switch statement?

Answer 17

It is appropriate to use a switch statement when you are testing an expression is based on a single integer, string, etc…as opposed to if-then-else which can be used with an expression based on ranges of values or conditions

Question 18

What does the throw statement do?

Answer 18

The throw statement raises an exception. If it is part of a try/catch block, execution is passed to the catch clause

Question 19

Describe an expression statement

Answer 19

An expression statement can: 1. assign values to one or more variables or members 2. invoke a method 3. delete an object property

Question 20

Define a JavaScript object

Answer 20

A JavaScript object is a mutable container of properties, where each property has a name and a value pair

Question 21

Demonstrate an object literal

Answer 21

var person = {

name :  'John',

age : 30

};

Question 22

JavaScript includes a _ feature that allows an object to _ the properties of _

Answer 22

JavaScript includes a prototype linkage feature that allows an object to inherit the properties of another object

Question 23

When is it appropriate to use quotes around the name of an object property

Answer 23

It is appropriate to use quotes around the name of an object property when the name has a space eg

var person = {

“first name” : ‘John’

}

Question 24

var animal = {

type : ‘dog’,

age : 2

}

console.log(animal.breed.breedType)

What is the outcome?

Answer 24

TypeError because there is no defined breed.breedType

Question 25

Demonstrate how objects are passed around by reference

Answer 25

var person = {

name : ‘Jane’,

age : 30 ,

gender : ‘female’

};

var jim = person;

jim. gender = ‘male’;
console. log(person.gender)

Outcome is male because person and jim reference the same object

Question 26

True or false, every object is linked to a prototype object from which it can inherit properties.

Answer 26

True, every object is linked to a prototype object from which it can inherit properties. All objects created from object literals are linked to Object.prototype, an object that comes standard with JS

Question 27

True or false, the protoype link has no effect on updating

Answer 27

True, the prototype link has no effect on updating, i.e. when an update is made on the inheriting object, the prototype object remains unchanged

Question 28

Describe the prototype chain

Answer 28

The prototype chain is a link used by JavaScript to search for an object’s specific property. If JavaScript does not find the property within the object itself, it goes up the link, looking for the property in the object’s prototype, until it goes all the way to Object.prototype

Question 29

The prototype relationship is dynamic. Explain

Answer 29

Any changes that are made to the prototype will be immediately visible in all objects that are dependent on the prototype

Question 30

True or false, the hasOwnProperty method looks at the prototype chain

Answer 30

False, the hasOwnProperty method does not look at the prototype chain, as a result it is useful when looking for properties that are specific to an object

Question 31

What is the for in statement used for?

Answer 31

The for in statement is used for looping over properties in objects, or elements in array

for ( variable in array | object ) {

 statements;

}

Question 32

Describe one way to minimize the use of global variables

Answer 32

One way to minimize the use of global variables is to create a single global variable for your application

var myApp = {}

myApp.person = {

name : ‘John’

age : 30

};

myApp.animal = {

type : ‘dog’,

breed : ‘ german sheppard’

}

Question 33

Describe the roles(s) of a function

Answer 33

JavaScript functions do the following:

1. form the base of JavaScript’s modular unit
2. enclose statements
3. encapsulate information
4. enable code reuse
5. enable code composition

Question 34

Objects produced from object literals are linked to _

Answer 34

Objects produced from object literals are linked to Object.prototype

Question 35

Function objects are linked to _ which in turn is linked to _

Answer 35

Function objects are linked to Function.prototype, which in turn is linked to Object.prototype

Question 36

Demonstrate a function literal

Answer 36

var myFunction = function (a, b) {

return a * b;

};

Question 37

What is an anonymous function

Answer 37

An anonymous function is a function defined without a name

Question 38

Invoking a function suspends the _ of the _ function, and passes control to the _ _

Answer 38

Invoking a function suspends the execution of the current function, and passes control to the new function

Question 39

In additon to the declared parameters, every function receives two additional parameters, _ and _

Answer 39

In addition to the declared parameters, every function receives two additional parameters, this and arguments.

Question 40

What are the four patterns of invocation in JavaScript

Answer 40

1. Method invocation
2. Function invocation
3. Constructor invocation
4. Apply invocation

Question 41

The this parameter’s value is determined by _ _

Answer 41

The this parameter’s value is determined by the invocation pattern

Question 42

True or false, there is no runtime error when the number of arguments passed do not match the number of parameters defined for a function

Answer 42

True, there is no runtime error when there is a mismatch between the number of arguments passed and the number of parameters defined for a function

Question 43

What happens when there are more arguments passed than are defined parameters for a function

Answer 43

The extra arguments are ignored if there are more arguments than defined function parameters

Question 44

What happens when there are fewer arguments passed to a function than there are parameters defined

Answer 44

The missing arguments will be replaced by undefined

Question 46

True or false, arguments passed to a function are not checked for type

Answer 46

True, there is no type checking on the argument values, any type of value can be passed to any parameter

Question 47

Describe and demonstrate post-increment assignment

Answer 47

Post-increment is when the input value is assigned after it is returned

var a = 120;

var b = a++;

console.log(a, b) // a = 121, b = 120

Question 48

Describe and demonstrate pre-increment

Answer 48

With pre-increment, the value is incremented and then returned

var a = 123;

var b = ++a;

console.log(a, b); // a = 124, b = 124

Question 49

What is a compound operator?

Answer 49

A compound operator is a combination of an operator and an assignment

var a = 10;

a += 5; // same as a = a + 5

console.log(a) // 15;

Question 50

How do you find out the type of a value or variable?

Answer 50

The typeof operator can be used to find the type of a value or variable.

Question 51

What data type does typeof return?

Answer 51

The typeof operator returns a string that represents the data type e.g.

“string”, “number”, “boolean”, “function”, “undefined”, “object”

Question 52

What is Infinity

Answer 52

Infinity is a special JavaScript value that is too big for JavaScript to handle

Question 53

What is the result of Infinity - Infinity

Answer 53

Infinity - Infinity = NaN

Question 54

var s = ‘1’;

s++;

console.log( s + typeof s);

What is the outcome?

Answer 54

var s = ‘1’;

s++;

console.log( s + typeof s); // 2 number

Question 55

var b = !!false;

console.log(b);

what is the outcome

Answer 55

var b = !!false;

console.log(b); // false

if you use double negation you will get the original value

Question 56

what are the results of the following

true && true

true && false

false && true

false && false

true || true

true || false

false || true

false || false

Answer 56

true && true - true

true && false - false

false && true - false

false && false - false

true || true - true

true || false - true

false || true - true

false || false - false

Question 57

What is the outcome?

true || (b = 6);

Answer 57

true || (b = 6); // true

Question 58

What is the outcome?

true && ( b = 6);

Answer 58

true && (b = 6); // 6

When JavaScript encounters a non-Boolean expression as an operand in a logical operation, the non-Boolean expression is returned as a result

Question 59

What is the outcome?

var mynumber = mynumber || 10;

Answer 59

mynumber = mynumber || 10 ; // 10, because mynumber was not

previously initialized

Question 60

What is the outcome?

var mynumber = 0;

mynumber = mynumber || 10;

Answer 60

var mynumber = 0;

mynumber = mynumber || 10; // 10

the outcome is 10 because JavaScript overwrites the previously initialized value

Question 61

Why is it recommended to use the === equality operator

Answer 61

It is recommended to use the === equality operator, because,

unlike the == operator, JavaScript does not convert the data

types to see if there is a match. With ===, there is only a match

if both operands are equal and of the same type

Question 62

What is the outcome?

console.log(myvar);

Answer 62

console.log(myvar); // ReferenceError : myvar is not defined

Question 63

What is the outcome?

console.log(typeof myvar);

Answer 63

console.log(typeof myvar); // “undefined”

typeof always returns a string name of the data type, in this case the data type is defined

Question 64

What is the outcome?

var myvar;

console.log(myvar);

Answer 64

var myvar;

console.log(myvar); // undefined

This is because, everytime you declare a variable without

initializing it, JavaScript automatically initializes it with the value

undefined

Question 65

var myvar = null;

console.log(typeof myvar);

Answer 65

var myvar = null;

console.log(typeof myvar); // “object”

Question 66

What is the outcome?

console.log(var i = 3 + undefined);

Answer 66

console.log(var i = 3 + undefined); // NaN

This is because undefined is not converted to a number

Question 67

What is the outcome?

console.log(var i = 1 + null);

Answer 67

console.log(var i = 1 + null); // 1

This is because null is converted to 0

Question 68

What happens if you add a new element to an array, but leave a
gap in the array?

Answer 68

if you add a new element to an array, but leave a gap in the array, those elements in between that dont exist, return undefined if they are accessed.

Question 69

How do you delete an element in an array?

Answer 69

You delete an element in an array by using the delete operator

var myArray = [1,2,3];

delete myArray[0];

console.log(myArray);// [undefined, 2, 3];

Question 70

Illustrate a good way to check if a variable is defined

Answer 70

var myVariable = “”;

if(typeof myVariable !== undefined) {

console.log(“variable exists”);

} else {

console.log(“variable does not exist”);

}

Question 71

Rewrite the following using the ternary operator

var myName = ‘Saki’;

var result = ‘’;

if(myName === ‘Saki’) {

result = "true";

} else {

result = “false”;

}

Answer 71

var myName = ‘Saki’;

var result = myName === ‘Saki’ ? ‘true’ : ‘false’;

Question 72

function sum(a,b) {

return a + b;

}

console.log(sum(2));

What is the outcome

Answer 72

function sum(a,b) {

return a + b;

}

console.log(sum(2)); // NaN

The outcome is NaN because you are trying to add a number,
2, to undefined

Question 73

In functions, what is arguments?

Answer 73

Arguments is a bonus parameter that consists of an array-like object containing the arguments passed to the function

Question 74

Demonstrate how the arguments parameter works

Answer 74

function sum() {

var i;

var result = 0;

var no_of_parameters = arguments.length;

for( i = 0; i < no_of_parameters; i++ ) {

 result = result + **arguments**[i];

}

}

console.log(sum(1,2,3)) ; // result is 6. arguments is an array-like object containing the arguments passed to the parameter

Question 75

What is the outcome?

console. log(parseInt(‘34’));
console. log(parseInt(‘4K’));
console. log(parseInt(‘K55’));

Answer 75

console. log(parseInt(‘34’)); // 34
console. log(parseInt(‘4K’)); // 4
console. log(parseInt(‘K55’)); // NaN

Question 76

What is the function isFinite() used for?

Answer 76

The function isFinite() is used to check whether a number is not an infinite number or NaN

e.g. isFinite(Infinity) // false

     isFinite(44); // true

Question 77

What is the function encodeURI( ) used for?

Answer 77

The function encodeURI( ) is used to return a usable url

  var url = '**my test.asp?name=ståle&car=saab**';

  console. log(encodeURI(url));
  * *my%20test.asp?name=st%C3%A5le&car=saab**

Question 78

What is the outcome?

var a = 50;

function myFunction( ) {

console.log(a);

var a = 2;

console.log(a);

}

myFunction( );

Answer 78

What is the outcome?

var a = 50;

function myFunction( ) {

console.log(a); // undefined - see explanation below

var a = 2;

console.log(a); // 2

}

myFunction( );

a is undefined because of JavaScript’s hoisting behavior. When a new function is executed, JavaScript moves all declared variables to the top of the function, regardless of where there are declared. The important thing to note that is, it is only the declaration that is hoisted to the top, not the assigned values.

Question 79

Illustrate function literal notation

Answer 79

function literal notation :

   var person = function( ) {

      return name;

   }

Question 80

Illustrate a function expression

Answer 80

function expression :

function ( ) {

 return name;

}

named function expression:

function **myFunction** ( ) {

 return name;

}

Question 81

What is a callback function?

Answer 81

A callback function is a function that is passed as a parameter to another function, and then called inside that other function

Question 82

Illustrate the use of a callback function

Answer 82

function getInput(a, b, callback) {

var number1 = a, 

      number2 = b;

callback(number1, number2);

}

//callback function

function processInput(num1, num2) {

  var result = num1 \* num2;

  console.log(result);

  return result;

}

getInput(4, 5, processInput); // 20

Question 83

When are callback functions most useful in javaScript?

Answer 83

For asynchronous execution, such as making HTTP requests. No need to wait for a response, you can implement a callback function to get the response, while continuing current function execution

Question 84

Demonstrate the use of an anonymous callback function

Answer 84

function getInputValues(a, b, callback) {

 var num1 =

}

Question 85

Demonstrate a self invoking function

Answer 85

(function(name) {

console.log(‘Hello ‘ + name); // Hello Jane

return;

}());

Question 86

Describe one good use of self-invoking functions

Answer 86

Self invoking functions are useful when you want to avoid creating extra global variables

( function () {

 // code

} ( ) ) ;

Question 87

Demonstrate the use of a private function

Answer 87

function outer (param) {

function inner( inputvalue) { // private function

 return inputvalue \* 5;

}

return inner( param );

}

Question 88

What are the advantages of using private functions?

Answer 88

1. You keep the global namespace clean
2. You can control access to your functions

Question 89

Illustrate a function that returns another function, and invoke the returned function immediately

Answer 89

function first ( ) {

console.log(‘first’);

return function returned ( ) {

console.log( 'returned' );

};

}

first( ) ( ); // second set of parenthesis invokes the second function

Question 90

What is a closure?

Answer 90

A closure can be defined as a function that keeps a link to its parent scope, even after the parent has returned

Question 91

Why are all functions considered closures?

Answer 91

All functions are considered closures because every function maintains access to the global scope, which is never destroyed

Question 92

Demonstrate a closure

Answer 92

function myName( firstName ) {

var message = “hello…” ;

function myFullName ( lastName ) {

  // inner function has access to outer function parameter

   return message + firstName + " " + lastName;

}

return myFullName; // return myFullName function

}

var name = myName( “Saki” ); // at this point, myName function has returned

name( “Jabs” ); // logs Hello…Saki Jabs. Even after outer function has returned, inner function still has access to the outer function variable and parameter

Question 93

In a function closure, the function maintains a reference to the scope where it was defined, not to the variables and their values found in the scope during the function definition, demonstrate.

Answer 93

function myNumbers ( ) {

 var num = 100;

 return {

   getNum : function ( ) {

     return num;

   }

  setNum : function (newNum) {

    num = newNum;

  }

}

}

var callMyNumbers = myNumbers( );

callMyNumbers.getNum( ) ; // logs 100

callMyNumbers.setNum(200);

callMyNumbers.getNum( ) ; // logs 200

Question 94

True or false, defining an array with [] is called array literal notation

Answer 94

True, defining an array with [] is called array literal notation

Question 95

Define object literal notation

Answer 95

Object literal notation is the definition of an object with curly braces

var myObject = { name : ‘Saki’ } ;

Question 96

What is the [[Put]] method?

Answer 96

The [[Put]] method is a method invoked by JavaScript everytime a property is initially added to an object.

Question 97

What does the [[Put]] method do?

Answer 97

The [[Put]] method creates a spot for a property in the object in which the property is defined

Question 98

What is the result of calling the [[Put]] method?

Answer 98

The result of calling the [[Put]] method is to creation of the own property on the object

Question 99

An _ _ indicates that the specific instance of an object owns that _

Answer 99

An own property indicates that the specific instance of an object owns that property

Question 100

When a value is assigned to an existing object property, the _ operation is called

Answer 100

When a value is assigned to an existing object property, the [[set]] operation is called

Question 101

When checking if a property exists, why is the following code is unreliable?

if ( person.age ) {

 //do something

}

Answer 101

The code below is unreliable because of JavaScript’s coercion effects. If the value is truthy, then it is true, however, if the value is falsey, then the result is false, even though the property exists. For example, if age is 0, then the result will be false, but the age property exists.

if ( person.age ) {

 //do something

}

Question 102

The _ operator looks for a property in a specific object and returns _ if it finds it

Answer 102

The in operator looks for a property in a specific object and returns true if it finds it

Question 103

What does the hasOwnProperty( ) method do?

Answer 103

The hasOwnProperty( ) method checks if a specific property exists on an object, and returns true if it does

Question 104

What is the difference between the in property and the hasOwnProperty method?

Answer 104

The difference between the in property and the hasOwnProperty method is that in property searches through the prototype chain for a property, and the hasOwnProperty method looks for only properties that belong to a specific object

var myObject = {

name : 'Saki',

age : 36

};

console. log(‘toString’ in myObject); // true
console. log(myObject.hasOwnProperty(‘toString’)); false

Question 105

What is the outcome?

var myObject = {

name : ‘Saki’,

age : 36

};

delete myObject.age;

console.log( myObject.age );

Answer 105

var myObject = {

name : ‘Saki’,

age : 36

};

delete myObject.age;

console.log( myObject.age ); // undefined

Question 106

How is the for-in loop used to enumate all enumerable object properties?

Answer 106

The for-in loop is used to enumate all enumerable object properties by assigning the property name to a variable

var property;

for( property in myObject ) {

console. log( “Name: “ + property );
console. log( “Value: “ + myObject[property] );

}

Question 107

Show an example of a data property

Answer 107

Example of a data property

var myObject = {

name : ‘Saki’

}

Data properties contain a value, like name in the above example

Question 108

What do accessor properties do?

Answer 108

Accessor properties define a function to call when the property is called (getter) and a function to call when the property is written ( setter )

Question 109

Illustrate the syntax to define an accessor property using an object literal

Answer 109

var myObject = {

_name : ‘Saki’,

get name( ) {

return this.\_name;

},

set name(value ) {

this._name = value;

}

} ;

console.log( myObject.name ); // Saki

myObject.name = ‘Try’;

console.log( myObject.name ); // Try

Question 110

When are accessor properties most useful

Answer 110

Accessor properties are most usefule when

1. you want a special behavior to occur when setting a property value initiates some special behavior
2. reading a value requires the calculation of the desired returned value

Question 111

What is the outcome?

var myObject = {

_name : ‘Saki’,

get name( ) {

 return this.\_name;

}

}

console.log( myObject.name(‘Try’);

Answer 111

var myObject = {

_name : ‘Saki’,

get name( ) {

 return this.\_name;

}

}

console.log( myObject.name(‘Try’);

In this instance, the _name property becomes read-only, and attempting to write to it will cause the following:

1. silently fail in nonstrict mode
2. throw an error in strict mode

Question 112

If a variable is expected to hold an object at a later point, it is advisable to initialize it with null, why?

Answer 112

If a variable is expected to hold an object at a later point, it is advisable to initialize it with null because:

1. it is easy to check if the variable is null or not, e.g.if (myVar != null) { … }
2. a null value is considered to be an empty object pointer

Question 113

In JavaScript, what does double parenthesis in a function call do?

Answer 113

Double parenthesis are used when calling a function that was returned in another function. Example

function foo( ) {

var myname = ‘SJ’;

return function( ) {

console.log(myname)

}

}

To call the returned anonymous function: foo( )( )