JavaScript

Question 1

Explain the difference between var, let, and const

Answer 1

Var is globally or locally scoped.
Const and let are block scoped.
Var and let variables can be updated later.
Const variables cannot be updated later.
Var can be re-declared.
Let and Const cannot be re-declared.
Const and let cannot be hoisted, while var can be.

Question 2

What are the different data types?

Answer 2

String, Number, BigInt, Boolean, Undefined, Null, Symbol, Object

Question 3

Explain hoisting

Answer 3

Hoisting is when all variable and function declarations are moved to the top of the scope.

Question 4

What’s the difference between “==” and “===”?

Answer 4

”==” only compares values whereas “===” compares values and types.

Question 5

Explain implicit type coercion

Answer 5

Implicit type coercion is the automatic conversion of a value from one data type to another.
Example: When a number is added to a string, the number is always converted to a string.

Question 6

Is JavaScript a statically or dynamically typed language?

Answer 6

JavaScript is a dynamically typed language meaning the type of the variable is checked during run-time instead of during compile-time.

Question 7

What is NaN?

Answer 7

NaN property indicates a value that is not a legal number.

Question 8

Explain passed by value and passed by reference

Answer 8

JavaScript passes primitive data types by value and non-primitive data types by reference.
Passing by value means that the operater allocates a new space in memory and returns the address.
Passing by reference means that the operator directly passes the location of the original variable without allocating new space in memory.

Question 9

What are the primitive and non-primitive types?

Answer 9

Primitive:
String, Number, BigInt, Boolean, Undefined, Null, Symbol
Non-Primitive:
Object

Question 10

What’s the difference between primitive and non-primitive types?

Answer 10

Primitive types can only store a single value.

Non-primitive types can store multiple and complex values.

Question 11

What is an Immediately Invoked Function?

Answer 11

It is a function that runs as soon as it is defined.

Question 12

Explain Higher Order Functions

Answer 12

Higher Order Functions are functions that either take other functions as arguments or return other functions.

Question 13

Explain “this” keyword

Answer 13

The “this” keyword refers to the object that the function is a property of and the value will always depend on the object that is invoking the function.

Question 14

Explain call(), apply(), and bind()

Answer 14

These are all predefined methods in JavaScript.
call() invokes a method by specifying the owner object and can take arguments (separated by commas).
apply() is similar to call, but takes arguments as an array.
bind() returns a new function where the value of “this” keyword will be bound to the owner object which is provided as a parameter.

Question 15

What is currying?

Answer 15

Currying is a technique to transform a function of arguments n to n function of one of less arguments.
Example:
function add (a) {
  return function(b){
    return a + b;
  }
}

add(3)(4)

Question 16

Explain Scope and Scope Chain

Answer 16

Scope determines the accessibility of variables and functions within your code.
Scope chain is JavaScript engine’s process of finding a variable within the code. It starts by trying to find the variable in local scope, then moves to outer scope, and finally moves to global scope. If the variable is still not found, it will throw a reference error.

Question 17

What types of scope are there and what are the differences?

Answer 17

Global Scope - variables and functions can be accessed from anywhere inside the code.
Function Scope - variables and functions can only be accessed within the function they have been declared.
Block Scope - Related to let and const. Var does not have block scope. Variables declared between the curly braces “{ }” can only be accessed within that block.

Question 18

Explain closures

Answer 18

Closure is the ability for a function to store a variable for further reference even after it is executed.

Question 19

What are object prototypes?

Answer 19

A prototype allows us to use properties and methods on an object even if the properties and methods do not exist on the current object.

Question 20

What are callbacks?

Answer 20

Functions that are used as an argument to another function are called callback functions.

Question 21

What is memoization?

Answer 21

Memoization is when the return value of a function is cached based on its parameters. It’s used for heavy-duty / expensive functions. If the same paramter is used, the cached result will be return instead of computing a new result to save time.

Question 22

What is recursion?

Answer 22

Recursion is when a function calls itself repeatedly until it arrives at a result.

Question 23

What is the use of a constructor function?

Answer 23

Constructor functions are used to create multiple objects which have similar properties and methods.

Question 24

What is DOM?

Answer 24

DOM stands for Document Object Model. It represents the objects that comprise the structure and content of a document on the web.

Question 25

What are arrow functions?

Answer 25

Arrow functions are a new and shorter way to declare functions. They were introduced in ES6.

Question 26

Where should the “script” tag be inserted into an HTML file?

Answer 26

It can be inserted in the head or the body section depending on when you want your JavaScript to load. Some will suggest always putting it at the bottom of the body section so that errors in your JavaScript don’t prevent other sections of your webpage from loading.

Question 27

What is a Promise?

Answer 27

A Promise is a JavaScript object that represents the eventual success or failure of an asynchronous operation and the resulting value. It acts as a proxy for a value that is not necessarily known when the promise is created.

Question 28

What is AJAX?

Answer 28

AJAX is the use of the XMLHttpRequest object that allows you to communicate with a server, exchange data, and update the page without having to refresh the page.

Question 29

What is a RESTful API?

Answer 29

To be considered a RESTful API, the application programming interface must adhere to 6 constraints:

1. Client-server (separation of user interface and data storage)
2. Stateless (Session state is kept entirely on the client)
3. Cacheable (Meaning the client cache has the right to reuse the response data for later, equivalent requests)
4. Uniform interface (Architechtual constraints are used to guide the behavior of components)
5. Layered system (Each component cannot “see” beyond the immediate layer with which it is interacting)
6. Code on demand (This is optional and means client functionality to be extended by downloading and executing code in the form of applets or scripts)

Question 30

What is the benefit of using async/await?

Answer 30

It allows you to perform other work while waiting for the result of a long running task and makes asynchronous code behave like synchronous code.

Question 31

What is the difference between asynchronous and synchronous code?

Answer 31

Synchronous code runs one task at a time and waits for each task to finish before moving onto the next task. Asynchronous code on the other hand, can move to another task before the previous task is finished.

Question 32

What is the difference between cookes, localStorage, and sessionStorage?

Answer 32

localStorage is saved to the browser and does not clear when the browser is closed.
sessionStorage is also saved to the browser, but only until the browser tab or window is closed.
cookies are initially stored locally, but are then transmitted to the server that requested them in the first place.

Question 33

What is prototypical inheritance?

Answer 33

It is the ability for one object to access properties and methods from another object. An example of this is creating an “Animal” object prototype with properties such as “numberOfLegs”, “species”, and “countryNativeTo”. Then creating a new object called “Dog” that inherits those properties from that prototype.

Question 34

How do you copy an array?

Answer 34

Although there are multiple ways to do this, a few ways are by using either Array.from(), the spread operator […], or array.slice().

Question 35

How do you compare two objects?

Answer 35

One way is by doing the following:
JSON.stringify(obj1) === JSON.stringify(obj2);
Another is to convert them to a string by doing the following:
Object.entries(obj1).toString() === Object.entries(obj2).toString();
This will only work if the objects’ data is in the same order.
A better and deeper way of comparing your objects is by using a function that first checks the length of the properties to see if they are the same, then use a for loop to loop through the properties, comparing each one.

Question 36

Why is asynchronous programming important to JavaScript?

Answer 36

Since JavaScript is originally synchronous, the use of callbacks, promises, and async/await have made it possible to have multiple operations running at the same time, making the application faster by not blocking functions and code further down in the call stack from running before a previous function has finished its operation.

Question 37

What is the difference between null, undefined, and undeclared?

Answer 37

Undeclared means that the variable has not been declared with an appropriate keyword.
Undefined is when a variable has been declared, but not been assigned a value.
Null is assigned to a variable intentionally and represents the absence of a value.

Question 38

What is the difference between .map() and .filter() and .forEach()?

Answer 38

.map() iterates over each item in an array, transforms the item, and returns a new array with the same number of items as the original array.
With .filter() you are required to return a true or false condition which, when met, returns the element(s) you are looking for.
A .forEach() iterates over the array and does something with the items in the array without returning anything.

Question 39

How do you check for undefined and null?

Answer 39

undefined:
if(typeof(value) === "undefined") {
  return "undefined"
}
null:
if(typeof(value) === "object" && !value) {
  return "null"
}

Question 40

How do you copy an object?

Answer 40

With the spread operator (…)