Sample Questions Flashcards
What are the standard objects available?
window & document
What is a primitive value?
A primitive value is a value that has no properties or methods
Which types (5) of primitive values are there?
String Number Boolean Null Undefined
Are primitive values mutable?
No, Primitive values are immutable (they are hardcoded and therefore cannot be changed).
Can JavaScript variables contain many values?
Yes. Objects are variables too. But objects can contain many values.
Object values are written as name : value pairs (name and value separated by a colon).
let person = {
firstName: "Ron" ,
lastName: "de Vries",
}
What are named values, in JavaScript objects, called?
Properties
How do you call an action that can be performed on objects?
Methods
What is an object literal?
You define and create an object in one statement
const person = {
firstName: "Ron",
lastName: "de Vries",
age: 35
}
How to add a new object?
Use the keyword new
const person2 = new Object();
person2. firstName = “Ron”,
person2. lastName = “de Vries”;
How are objects addressed?
By reference, not by value
const x = person; // Will not create a copy of person. X is not a copy of person, it is person
What are some flavors of JavaScript?
TypeScript
CoffeeScript
JScript (Microsoft)
What is isomorphic JavaScript?
Code that can be run either on the client or server side
Which primitive data type has been added in ES6?
symbol
Why is it dangerous to use document.write?
If it’s used after the page has rendered (loaded) it will delete all content
What is returned if user cancels the prompt() ?
Null
What does the confirm() method return?
A boolean value; ok = true, cancel = false
What is block scope?
Variables declared inside a code block can’t be accessed outside of that block
What are the logical operators?
&& (AND), || (OR), ! (NOT)
What is the result if you add a number and a string?
String
What is the difference between a function declaration and an function expression?
Function declarations load before any code is executed. Expressions load only when the interpreter reaches that line of code
How does the Ternary Operator look like? Also called Conditional operator
x = myBoolean ? “Ok” : “Not ok”;
It takes three expression. If statement is true (x = myBoolean) “Ok” will be executed. If fasel, “Not ok” will be printed
What does the NOT (!) operator return
True for false statements
False for true statements
When comparing two string, what will be greater…. 2 or 12?
2 will be greater, because (alphabetically), 1 is less then 2. JS always looks at the first number
How to secure a proper result when comparing variables?
Convert them into the proper type before comparison
How to hide an HTLM element using JavaScript?
Set the display style to none
document.getElementById(“demo”).style.display = “none”;
What is the difference between a programming language and a scripting language?
Programming languages are compiled, whereas scripting languages are interpreted.
What is JQuery?
jQuery was designed to focus on client-side scripting. It is an open-source JavaScript library released under the MIT license. It simplifies the syntax of DOM manipulation and eliminates cross-browser incompatibilities
What was server-side JavaScript (SSJS) originally called?
Livewire
What was the original name for JavaScript
Mocha, then Livewire, then JavaScript
What is VB Script?
VBScript is based on the full programming language, Visual Basic. It was developed by Microsoft. Unlike VBScript, JScript and ECMAScript are flavors of JavaScript.
What describes an object-oriented program?
A collection of individual objects that perform different functions.
Which three pop-up boxes are there?
alert, confirm, prompt
Where does JIT stand for?
Just In Time
Some programming languages? (Compiled)
C++
Java
Python
What is an object?
Collection of properties and methods
Define a car object with a drive method
const car = {
type: "Fiat",
model: "500",
color: "white",
drive: function () {
return "drive"
} };Promp returns…
User closes window –> NULL
User enters string + OK –> STRING
No value entered but OK click –> EMPY STRING
All arithmic operator become assignment operators when….
The arithmic operator is placed in front of the assignment operator
Example:
+=
+ is arithmic
= is assignment
+= will resolve in an assignment operator
What applies ‘’ the internal state of a program’’ ?
variables
When a function produces a value it is said to …..
return it
While / Do loop difference?
Do loop will execute it’s body at least once and only then start testing whether it should continue
Syntax of a For Loop?
for ( let i = 0, i < string.length; i ++ ) { }
1st parameter = initiate the loop
2nd parameter = expression check if loop should run
3rd parameter = counter (mostly ++ indicating to add 1 each iterarion)
Is JavaScript case-sensitive?
Yes
Selft contained set of related values and functions
Object
Example of a type attribute?
type = “application/javascript’’
What should a variable name start with?
Upper or lower case LETTER
$
Underscore
Give an example of assigning a variable
firstName = “Ron”;
What to use to fint out the data type?
typeof
Example:
console.log(typeof variablename);
What is A += B shorthand for? += also known as the Compound Assignment Operator
A = A + B;
A snipped of code that evaluates to a value?
Expression
What is type coercion?
Two different date types are concatenated. Solution: change date types appropriately (CASTING)
Type Conversion
Converting Strings to Numbers Converting Numbers to Strings Converting Dates to Numbers Converting Numbers to Dates Converting Booleans to Numbers Converting Numbers to Booleans
Some Number() methods
Number() Returns a number, converted from its argument
parseFloat() Parses a string and returns a floating point number
parseInt() Parses a string and returns an integer
Some Date methods
getDate() Get the day as a number (1-31)
getDay() Get the weekday a number (0-6)
getFullYear() Get the four digit year (yyyy)
getHours() Get the hour (0-23)
getMilliseconds() Get the milliseconds (0-999)
getMinutes() Get the minutes (0-59)
getMonth() Get the month (0-11)
getSeconds() Get the seconds (0-59)
getTime() Get the time (milliseconds since January 1, 1970)
Converting Booleans to Numbers
Number(false) // returns 0
Number(true) // returns 1
What’s the difference between putting the ++ operator before OR after the variable?
Main difference is the value that is returned. Points++ will return the original value THEN increase it by 1. ++points will increase FIRST and then return the new value.
Example: let points = 2; points ++ // Will return 2, then increase points to 3 \++points // Will increase to 3 and then return
What happends if number is too big or too small?
5e-324 will be return OR inifinity
How to check if a value is a number?
Use the Number.isFinite() method.
True = value is a number
Example:
Number.isFinite(42)
—> true
“2”+ 8 will return….
28
Type Coercion
JS will convert the number to a string by default
Difference Undefine and NULL
Undefined is the value given to variables that have not been assigned a value. NULL means ‘no value’. It can be thought of as a placeholder. Both are non-value types.
Example:
10 + null // null behaves like zero
–> 10
10 + undefined // undefined is not a number
–> NaN
In general, values tend to be set to undefined by JS, whereas values are usually set to NULL manually by the coder
Where does the this keyword point to in a function definition?
It refers to the ‘owner’ of the function
What is the call(() method?
With call(), an object can use a method belonging to another object.
Example
const person = {
fullName: function() {
return this.firstName + " " + this.lastName;
}
}
const person1 = {
firstName:"John",
lastName: "Doe"
}
const person2 = {
firstName:"Mary",
lastName: "Doe"
}
// This will return "John Doe": person.fullName.call(person1);
What is the apply() method?
With the apply() method, you can write a method that can be used on different objects. The apply() method is similar to the call() method
Examples of HTML events
An HTML web page has finished loading
An HTML input field was changed
An HTML button was clicked
Example of HTML event + handler
The time is?
Common HTML events
onchange An HTML element has been changed
onclick The user clicks an HTML element
onmouseover The user moves the mouse over an HTML element
onmouseout The user moves the mouse away from an HTML element
onkeydown The user pushes a keyboard key
onload The browser has finished loading the page
The onload and onunload Events
The onload and onunload events are triggered when the user enters or leaves the page.The onload event can be used to check the visitor’s browser type and browser version, and load the proper version of the web page based on the information.
The onload and onunload events can be used to deal with cookies.
Recommended method of Event Listeners
document.getElementById(“myBtn”).addEventListener(“click”, displayDate);
addEventListener()
The addEventListener() method attaches an event handler to the specified element.
The addEventListener() method attaches an event handler to an element without overwriting existing event handlers.
You can add many event handlers to one element.
Syntax of addEventListener
element.addEventListener(event, function, useCapture);
What is Event bubbling? And what is Event capturing?
In bubbling the inner most element’s event is handled first and then the outer
In capturing the outer most element’s event is handled first and then the inner
DEFAULT = Bubbling
removeEventListener()
element.removeEventListener(“mousemove”, myFunction);
Difference between call() and apply()
Bij de call() method voert u de argumenten afzonderlijk in, bij de apply() method als array:
person. fullName.call(person1, “Oslo”, “Norway”);
person. fullName.apply(person1, [“Oslo”, “Norway”]);
What is the global object?
n de context van client-side JavaScript is het window object het global object. Vanaf ES2020 kan naar dit object verwezen worden met globalThis.
JS global properties
Infinity A numeric value that represents positive/negative infinity
NaN “Not-a-Number” value
undefined Indicates that a variable has not been assigned a value
Global Functions
decodeURI() Decodes a URI
decodeURIComponent() Decodes a URI component
encodeURI() Encodes a URI
encodeURIComponent() Encodes a URI component
escape() Deprecated in version 1.5. Use encodeURI() or encodeURIComponent() instead
eval() Evaluates a string and executes it as if it was script code
isFinite() Determines whether a value is a finite, legal number
isNaN() Determines whether a value is an illegal number
Number() Converts an object’s value to a number
parseFloat() Parses a string and returns a floating point number
parseInt() Parses a string and returns an integer
String() Converts an object’s value to a string
unescape() Deprecated in version 1.5. Use decodeURI() or decodeURIComponent() instead
Changing variables from one data type to another?
Casting
What is an argument in JavaScript?
A value passed into a function from outside the function.
Consider the following JavaScript function:
function myFunction(x) {
//Insert Code Here
}
Which code statement should be added to the function to return to the value of x multiplied by 2 to the calling statement?
return x * 2
Consider the following HTML code:
Form
<div>Div
<p><br></br>Paragraph</p></div>
The onlick event is first captured by the element. Event capturing starts at the highest level in the DOM hierarchy tree (generally, the Window object).
Consider the following code block:
function sample (fname, lname) {
return fname + lname;
}
document.write(sample(prompt("Please enter your first name:"), prompt("Please enter your last name:")));
Output?
SamSmith
Which function is created at runtime and declared without a name?
Anynymous functions
What will be the output of the following code snippet?
var x = 10, y = "10.1x", z = "15xy"; var a = x + parseInt(y) + parseFloat(z); document.write(a);
The parseInt(y) method returns 10, and the parseFloat(z) method returns 15. Output will be 35
Which JavaScript property is used to add new properties and methods to objects?
Prototype
When does the load event occur?
When a page finishes loading
How does a global variable differ from a local variable?
You can access a global variable value from any function or block that you define on the HTML page.
9 + true = ……
10
AWhen number is added to Boolean, the boolean is cast into a number first. True as 1 and false as 0
Are Anynomous functions hoisted in JS?
No
Let and const (hoisting)
Variables defined with let and const are hoisted to the top of the block, but not initialized.
Which form element would you use to create a scriptable button other than a reset or submit button?
button
The __________ method tests for the presence and position of a certain character.
indexOf()
ServerSide JavaScript will not function if LiveWire is not installed.
True
A __________ is an organized block of code that handles actions generated by user events.
statement
Assuming Z= 3 and y= 2, what would be the result of : Z+=Y
5
The name=value pair is the only information required to generate a cookie. All other parameters are optional.
True
The select object has three event handlers: __________, __________ and __________.
onBlur, onFocus, onChange
………… are used in expressions to store or return a value…..
Operators
An Instance is the term for the real-time objects that are generated from the empty constructor template?
True
To call the alert() method, you need only place a simple line of code, called a ___________, in a script block somewhere in your document.
statement
A ______________ is used to send form contents to a server.
submit button
The button object is the simplest of all objects. Its main event handler is….
onClick
In JavaScript, there are __________ types of expressions.
4
The main event handler associated with the radio button object is _________.
onClick
__________ are the actual data values you provide in JavaScript.
Literals
What would be the correct JavaScript way to make a text string appear in bold print?
“Hello!”.bold();
After calling the constructor function, the keyword “constructor” returns the specific properties for the custom object to the calling instantiation statement.
False
The term __________________ refers to any application, such as a Web Browser or help engine, that renders HTML for display to users.
user agent
With JavaScript, it is possible to change two frames simultaneously. To do this, you must write a function that includes _________________
two location.href statements
Variables can store _______ data types in JavaScript.
5
A _________ creates an empty template from which real-time objects, called __________, can be generated.
constructor, instances
The String, Array, Date and Math objects are all ____________ objects.
language
alinkColor, anchors, bgColor, cookie and fgColor are all properties of the _______________ object.
document
Examples of errors
Syntax error: alert(“Hi); // missing “
Runtime error: let myNumber = 42;, mynumber ++; // myNumber not defined
Logical error: concatenation instead of addition
What is a typical watchpoint?
alert()
What type of error is difficult to debug?
Logical error
Consider the following code block:
var a = "10", b = 20;
document.write( a + b );
1020
Which type of error are you most likely to debug using watchpoints?
Logical error
Which type of error occurs after the script has loaded and is running?
Run-time error
Which type of error causes no error alert in the browser?
Logical
Identify the type of error in the following code block (if any):
document.write(“Heard of the Higgs Boson?”;
Syntax (load-time) error because the code block has a missing closing parenthesis
Which type of error is typically caused by improper use of commands and usually causes an error alert?
Run-time errors
Identify the type of error in the following code snippet (if any):
document.writenl(“JavaScript code has hidden errors!”);
The statement contains a run-time error. The statement is syntactically correct, but it is trying to call the writenl method that does not exist. The correct statement which will run successfully is:
document.write(“JavaScript code has hidden errors!”);
