Javascript exam study

What is an event loop?

it powers javascript’s async behavior

Where does javascript keep track of function calls?

Call stack

A call stack follows a ______ order

first in first out

What erro do you get from a stack being too large?

stack overflow

Where does memory allocation occur?

in the heap

where are objects, variables, and function declarations stored?

in the heap

What are not part of the Javascript API and handle network requests?

APIs

When async operations are completed, their callblacks are placed where?

in the callback/task queue

callback/task queues follow a ____ order

first in first out

where are macrotasks contained?

Callback/task queues

What higher priority queue is used for certain types of calbacks?

Microtask queue

Microtask queues follow a _____ order

first in first out

What mechanism continuously checks the call stack and queues?

event loop

What is the 1st step in executing async code?

Execution phase

What is the 2nd step in executing async code?

Delegation phase

What is the 3rd step in executing async code?

Completion phase

What is the 4th step in executing async code?

Event loop processing phase

Which has a higher priority: microtasks or macrotasks

microtasks

Macro tasks are processed after ______

all the microtasks have been processed

What is this a diagram of?

Event loop execution

What is this box?

Microtask queue

What is this?

microtask

What is this box?

call stack

what is this?

callback

What is this box?

Web APIs

What is this?

enqueue

What is this box?

Macrotask queue

what is this?

macrotask

What are callbacks?

functions passed as arguments to another function

What is a synchronous callback?

callback functions that are executed immediately

What is an asynchronous callback?

A callback function that is executed later

What is an error-first callback?

a callback function function reserved for errors

What is a promise?

objects representing completion/failure of async operations

What is a pending promise?

the initial state of a promise when the operation is in progress

What is a fulfilled promise?

when the operation is successful and the promise has a resulting value

What is a rejected promise?

When the operation has failed and the promise has the cause of that failure

What does the following code do?

const promise = new Promise((resolve, reject) => {
    if (/* operation successful */) {
        resolve(value); 
    } else {
        reject(reason); 
    }
});

Creates a promise

What does this method do?

promise.then(
  (value) => { /* fulfillment handler */ },
  (reason) => { /* rejection handler (optional) */ }
);

registers callbacks for fulfillment and/or rejection

What does this method do?

promise.catch((reason) => { /* rejection handler */ });

registers a callback for rejection

What does this method do?

promise.finally(() => { /* cleanup code */ });

registers a callback to be fulfilled regardless of promise value

What does promise.resolve(value) do?

returns the resolved promise with the given value

What does promise.reject(reason) do?

returns the rejected promsie with the given reason

What does the promise.all(iterable) do?

returns a promise that is resolved when all the promises within the iterable are resolved

What does the promise.race(iterable) do?

returns a promise that resolves or rejects as soon as one of the promises within the iterable resolves or rejects

What does the promise.allSettled(iterable) do?

returns a promise that resolves when all promsies within the iterable have settled

What does the promise.any(iterable) do?

returns a promise that resolves as soon as any of the promises in the iterable resolves

What does async mean?

an async function that return a promise

What does await mean?

paues the executions of a async function until a promise is settled

What does the following code do?

async function myAsyncFunction() {
    return 'Hello World';  
}

Creates an async function

What does the following code do?

async function fetchUserData() {
    try {
        const response = await fetch('/api/user'); 
        const userData = await response.json(); 
        return userData; 
    } catch (error) {
        
        console.error('Error fetching user data:', error);
        throw error; // Re-throw to propagate the error
    }
}

Creates an async function with await

Always wrap await expression in a ______

try-catch block

What is data serialization?

converting structured data to a format that allows sharing or storage

What step 1 in data serialization?

Data collection

What step 2 in data serialization?

Conversion

What step 3 in data serialization?

Transmission

What step 4 in data serialization?

Storage or processing

What does the following code do?

const person = {
  name: "John Doe",
  age: 30,
  isStudent: false,
  courses: ["Web Development", "Database Design"]
};

const serializedData = JSON.stringify(person);
console.log(serializedData);

serializes an object literal into a JSON string

What is the output of the following code?

const person = {
  name: "John Doe",
  age: 30,
  isStudent: false,
  courses: ["Web Development", "Database Design"]
};

const serializedData = JSON.stringify(person);
console.log(serializedData);

{"name":"John Doe","age":30,"isStudent":false,"courses":["Web Development","Database Design"]}

What is deserializing?

converting serialized data back into it;s OG structure like an object or array

What is step 1 in deserialization?

data reception

What is step 2 in deserialization?

Validation

What is step 3 in deserialization?

Conversion

What is step 4 in deserialization?

utilization

What does this code do?

const serializedData = '{"name":"John Doe","age":30,"isStudent":false,"courses":["Web Development","Database Design"]}';

try {
  const person = JSON.parse(serializedData);

  console.log(person.name);
  console.log(person.courses[0]);
} catch (error) {
  console.error("Deserialization error:", error);
}

Deserializes a JSON string back into an object

What is the output of

const serializedData = '{"name":"John Doe","age":30,"isStudent":false,"courses":["Web Development","Database Design"]}';

try {
 
  const person = JSON.parse(serializedData);

  console.log(person.name);
  console.log(person.courses[0]);
} catch (error) {
  console.error("Deserialization error:", error);
}

John Doe
Web Development

What are CSV files?

Comma seperated values with rows and columns

What data format is the following:

name,age,email,role
John Doe,30,[e-mail],Developer
Jane Smith,28,[e-mail],Designer
Mike Johnson,35,[e-mail],Manager

CSV

What does the following code do?

function parseCSV(csv) {
  const lines = csv.split('\n');
  const result = [];
  const headers = lines[0].split(',');

  for (let i = 1; i < lines.length; i++) {
    const obj = {};
    const currentLine = lines[i].split(',');

    for (let j = 0; j < headers.length; j++) {
      obj[headers[j]] = currentLine[j];
    }

    result.push(obj);
  }

  return result;
}

const csvData = `name,age,email,role
John Doe,30,[e-mail],Developer
Jane Smith,28,[e-mail],Designer`;

const parsedData = parseCSV(csvData);
console.log(parsedData);

parses CSV data

What is XML?

eXtensible Markup Language that has a hierarchal structure and looks a but liek HTML

What data format is the following:

<?xml version="1.0" encoding="UTF-8"?>
<employees>
  <employee id="1">
    <name>John Doe</name>
    <age>30</age>
    <email>[e-mail]</email>
    <role>Developer</role>
  </employee>
  <employee id="2">
    <name>Jane Smith</name>
    <age>28</age>
    <email>[e-mail]</email>
    <role>Designer</role>
  </employee>
</employees>

XML

What does the following code do?

import { JSDOM } from 'jsdom';

function parseXML(xmlString) {

const dom = new JSDOM(xmlString, { contentType: 'text/xml' });

return dom.window.document;

}

const xmlString = `<?xml version="1.0" encoding="UTF-8"?>

<employees>

<employee id="1">

<name>John Doe</name>

<age>30</age>

</employee>

<employee id="2">

<name>Jane Smith</name>

<age>28</age>

</employee>

</employees>`;

const xmlDoc = parseXML(xmlString);

const employees = xmlDoc.getElementsByTagName("employee");

console.log("Employee Information:");

for (let i = 0; i < employees.length; i++) {

const name = employees[i].getElementsByTagName("name")[0].textContent;

const age = employees[i].getElementsByTagName("age")[0].textContent;

const id = employees[i].getAttribute("id");

console.logID: ${id}, Name: ${name}, Age: ${age});

}

Parses an XML file

What is JSON?

JavaScript Object Notation that uses key-value pairs

What data format is the following:

{
  "employees": [
    {
      "id": 1,
      "name": "John Doe",
      "age": 30,
      "email": "[e-mail]",
      "role": "Developer"
    },
    {
      "id": 2,
      "name": "Jane Smith",
      "age": 28,
      "email": "[e-mail]",
      "role": "Designer"
    }
  ],
  "company": "Example Corp",
  "established": 2010
}

JSON

What does the following code do?

const jsonString = '{"name":"John","age":30,"city":"New York"}';
const person = JSON.parse(jsonString);
console.log(person.name);

const team = {
  name: "Development Team",
  members: ["John", "Jane", "Mike"],
  lead: {
    name: "John Doe",
    yearsExperience: 5
  }
};

const teamJSON = JSON.stringify(team);
console.log(teamJSON);

Parses JSON data

What data format is tabular and godo for spreadsheets, data exports?

CSV

What data format is good for complex data with metadata, document based apps, and enterprise systems?

XML

What data format is good for web APIS, configuration files?

JSON

In JSON, data is represented using _______

key value pairs

What does JSON.Stringify() do?

converts JavaScript objects into JSON strings

What does the following code do?

const person = {
  name: "John Doe",
  age: 30,
  isStudent: false
};

const jsonString = JSON.stringify(person);
console.log(jsonString);

Converts the object into a JSON string

What is the output of

const person = {
  name: "John Doe",
  age: 30,
  isStudent: false
};

const jsonString = JSON.stringify(person);
console.log(jsonString);

{"name":"John Doe","age":30,"isStudent":false}

What does JSON.parse() do?

converts a JSON string into a JavaScript object

What does the following code do?

const jsonString = '{"name":"John Doe","age":30,"isStudent":false}';

const person = JSON.parse(jsonString);

console.log(person.name);
console.log(person.age);
console.log(typeof person.isStudent);

converts a JSON string into an object

What is the output of the following

const jsonString = '{"name":"John Doe","age":30,"isStudent":false}';


const person = JSON.parse(jsonString);

console.log(person.name);
console.log(person.age);
console.log(typeof person.isStudent);

John Doe
30
boolean

What is the difference between JSON and dictionaries?

JSON is a file format that uses key value pairs while dictionaries are data types

What does the following code do?

const user = {
  name: "Alice",
  password: "secret123", 
  email: "[e-mail]",
  loginAttempts: 5
};

function replacer(key, value) {
  if (key === "password") {
    return undefined; 
  }
  return value;
}

const safeUserJson = JSON.stringify(user, replacer);
console.log(safeUserJson);

Uses a replacer function to exclude the password value

What is the output of the following?

const user = {
  name: "Alice",
  password: "secret123", 
  email: "[e-mail]",
  loginAttempts: 5
};


function replacer(key, value) {
  if (key === "password") {
    return undefined; 
  }
  return value;
}

const safeUserJson = JSON.stringify(user, replacer);
console.log(safeUserJson);

{"name":"Alice","email":"[e-mail]","loginAttempts":5}

What does the following code do?

const user = {
  name: "Bob",
  age: 25,
  email: "[e-mail]",
  phone: "555-1234",
  address: "123 Main St"
};


const properties = ["name", "email"];
const filteredJson = JSON.stringify(user, properties, 2);
console.log(filteredJson);

uses a replacer function to only include the name and email

What is the output for

const user = {
  name: "Bob",
  age: 25,
  email: "[e-mail]",
  phone: "555-1234",
  address: "123 Main St"
};

const properties = ["name", "email"];
const filteredJson = JSON.stringify(user, properties, 2);
console.log(filteredJson);

{
  "name": "Bob",
  "email": "[e-mail]"
}

What are the unsupported data types in JSON?

functions, date objects, circular references, and symbols

What does the following code do?

const jsonWithDate = '{"name":"Meeting","date":"2025-07-11T09:00:00.000Z"}';


function dateReviver(key, value) {
  if (key === "date" && typeof value === "string") {
    return new Date(value);
  }
  return value;
}

const meeting = JSON.parse(jsonWithDate, dateReviver);
console.log(meeting.date instanceof Date);
console.log(meeting.date.toLocaleString());

uses a reviver function to convert date strings into date objects