Basics

Question 1

How do you add a JS script to a HTML page?

Answer 1

<script type="text/javascript">
    // Your code goes here ...
</script>

Question 2

How do you include an external JS file in an HTML page?

Answer 2

<script src="path/to/filename.js"></script>

Question 3

How do you change the contents within a HTML element with id=’target’, with a button click?

Answer 3

<button tyle='button' onclick=' document.getElementById("target").innerHTML = "Hello World!" '>Click Me!</button>

Question 4

How do you change the font size of an element with id=’target’?

Answer 4

document.getElementById("target").style.fontSize = "32px";

Question 5

How do you hide an element with id=’target’?

Answer 5

document.getElementById("target").style.display = "none";

Question 6

Write a basic JS function to change the contents of an element with id=’target’.

Answer 6

<script>
function myFunction() {
  document.getElementById("target").innerHTML = "Hello World!";
}
</script>

Question 7

How do you call an elfunction with a button click?

Answer 7

<button type="button" onclick="myFunction()">Try it</button>

Question 8

How do you trigger an alert box to pop up?

Answer 8

alert('This is an alert!')

or

window.alert('This is also an alert!')

Question 9

Create a prompt to get someone’s age

Answer 9

let age = prompt("What's your age?")

Question 10

What is duck typing?

Answer 10

If it looks like a duck and sounds like a duck it must be a duck!

Question 11

How can you find out the type of a variable

Answer 11

use the typeof command. For example:

var a = 10;
console.log(typeof a)

>>> number

Question 12

What will ‘typeof null’ return?

Answer 12

>>> object

There is a bug in JS that they can’t fix (without breaking a lot of legacy code). It returns object and not null (it is a primitive, not a reference type).

Question 13

What is the difference between == and ===?

Answer 13

== uses type coercion.
=== compares for equality but without any type coercion.

Question 14

What is type coercion

Answer 14

Type coercion is where a variable type is change by JS automatically. For example using a number as a boolean.

Question 15

What is block scope?

Answer 15

It is anything contained within curly braces {}
This is used a lot, if for example if statements.

Question 16

What is function scope

Answer 16

function scope is anything defined within the function

Question 17

What is the difference between block and function scope?

Answer 17

Variables created within a block scope are available to the parent scope, this is not the case for function scope.

Question 18

What is the heap?

Answer 18

A mostly unstructured area of memory where JS objects are stored

Question 19

What is the stack?

Answer 19

it is the JS que, a todo list of code to execute. Each frame in the stack represents a function call in the code being executed.

Question 20

How do browsers and APIs enhance JS?

Answer 20

Browsers and web API’s are not part of the JS language, but provide extensions to what you can do with the language.

Question 21

What is the task queue?

Answer 21

The task queue is part of the browser, and is responsible for managing the stack and the task queue. There is a seperate render queue for the display.

Question 22

What is blocking?

Answer 22

Code that is slow, like network requests or image processing, can hold up the processing of frames in the stack and prevent the user from interacting with the browser.

Asynchronous callbacks are the answer to browser blocking.

Question 23

Write a turnery operator statment

Answer 23

let food = 'bread';

let breakfast = (y === 'bread') ? 'toast' : 'no toast';

Question 24

What is an IIFE?

Answer 24

It’s an ‘Immediately Invoked Function Expression’

Putting a function in braces makes it an expression, and puting an empty set of braces after it tells JS to call or invoke the function immediately

(favNumber = function (num = 3) {

   console.log("my favorite number is " + num);

})();

favNumber(10);

This code will return twice, once with 3 and once with 10.

Question 25

What are the main JS engines?

Answer 25

• V8 - The Chrome engine and nodejs
• Spidermonkey - Firefox
• Chakra - Microsoft and there version of node
• JSC - JavaScriptCore was initially built for webkit and now powers Safari and React Native Applications.

You can play with these engines directly using jsvu at the command line

Question 26

How do JS engines work?

Answer 26

All the JS engines follow the same basic process but have different systems for the optimisation.

The source code is fed into the parser, and this creates the abstract syntax tree. This is fed into the interpreter which optimises and executes the code.

V8 has one optimiser, Spidermonkey and Chakra both have two, and JSC has 3 code optimisers!

V8 is written in C++, and the code is open source. Thus, you can expand on its codebase if you want to.

Question 27

How do bitwise operations work?

Answer 27

The bitwise operator goes though each column comparing values.

console.log (1 | 2);

will compare 00000001 or 00000010. The result is 00000011, or 3,.

Question 28

What are the logical operators for OR and AND?

Answer 28

OR is

||

AND is

&&

The bitwise operators for OR and AND are simply

|

and

&

Question 29

What are the three basic data types in JavaScript?

Answer 29

strings, numbers, and booleans

Question 30

How do you declare a variable in JavaScript?

Answer 30

You can declare a variable in JavaScript using the “var”, “let”, or “const” keywords, followed by the variable name and an optional initial value.

Question 31

What is the difference between let and var in JavaScript?

Answer 31

The main difference between “let” and “var” in JavaScript is that “let” has block scope, while “var” has function scope.

Question 32

How do you concatenate strings in JavaScript?

Answer 32

You can concatenate strings in JavaScript using the “+” operator.

Question 33

What is the difference between == and === in JavaScript?

Answer 33

The “==” operator checks for equality in value, while the “===” operator checks for equality in value and data type.

Question 34

How do you create an if statement in JavaScript?

Answer 34

You can create an if statement in JavaScript using the “if” keyword, followed by a condition in parentheses and the code to execute if the condition is true in curly braces.

Question 35

What is a for loop and how do you use it in JavaScript?

Answer 35

A for loop is a control flow statement that allows you to execute a block of code multiple times. You can use a for loop in JavaScript by using the “for” keyword, followed by a variable declaration, a condition, and an optional increment or decrement.

Question 36

What is a function and how do you define one in JavaScript?

Answer 36

A function is a block of code that can be called and executed at any time. You can define a function in JavaScript using the “function” keyword, followed by the function name, parameters in parentheses, and the code to execute in curly braces.

Question 37

How do you access an element in the HTML document using JavaScript?

Answer 37

You can access an element in the HTML document using JavaScript by using the “document” object and methods such as “getElementById”, “getElementsByClassName”, or “getElementsByTagName”.

Question 38

What is an array and how do you create one in JavaScript?

Answer 38

An array is a data structure that can store multiple values in a single variable. You can create an array in JavaScript by using square brackets, with each value separated by a comma.

Question 39

What is a callback function in JavaScript?

Answer 39

A callback function is a function that is passed as an argument to another function and is executed when a specific event or condition occurs.

Question 40

What is the difference between null and undefined in JavaScript?

Answer 40

Null is a value that represents the intentional absence of any object value, while undefined is a value assigned to variables that have not been assigned a value.

Question 41

What is the difference between a while loop and a do-while loop in JavaScript?

Answer 41

A while loop first checks the condition and then executes the code, while a do-while loop executes the code first and then checks the condition.

Question 42

What is the ternary operator in JavaScript?

Answer 42

The ternary operator is a shorthand way of writing an if-else statement that returns a value based on a condition.

Question 43

What is a template literal in JavaScript?

Answer 43

A template literal is a string literal that allows you to embed expressions and variables inside the string using backticks (`).

Question 44

How do you add an event listener to an element in JavaScript?

Answer 44

You can add an event listener to an element in JavaScript using the “addEventListener” method and specifying the event type and function to execute when the event occurs.

Question 45

What is a constructor function in JavaScript?

Answer 45

A constructor function is a function that is used to create new objects with the same properties and methods.

Question 46

What is the spread operator in JavaScript?

Answer 46

The spread operator is a syntax that allows you to spread the elements of an array or object as separate arguments or elements.

Question 47

What is hoisting in JavaScript?

Answer 47

Hoisting is a mechanism in JavaScript that moves variable and function declarations to the top of their respective scopes.

Question 48

What is the difference between let and const in JavaScript?

Answer 48

The main difference between “let” and “const” in JavaScript is that “let” variables can be reassigned, while “const” variables are read-only and cannot be reassigned.

Question 49

What is the difference between synchronous and asynchronous programming in JavaScript?

Answer 49

Synchronous programming is when code is executed in a sequential manner, while asynchronous programming allows code to be executed in a non-sequential manner without blocking the execution of other code.

Question 50

What is a promise in JavaScript?

Answer 50

A promise is an object that represents the eventual completion (or failure) of an asynchronous operation and allows you to attach callbacks that will be executed when the operation is completed (or failed).