Lesson 3

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Data Types

• String: Text values enclosed in single or double quotes. Example: 'mama'
• Number: Numeric values, integers, or decimals. Example: 10
• Boolean: Represents true or false values. Example: isPaid = true
• Null: Represents "nothing"; explicitly set by the developer. Example: obj.name = null
• Undefined: Represents "nothing"; not explicitly set, but rather assigned by JavaScript. Example: let variable;

Special Values

• NaN (Not a Number): Result of invalid mathematical operations. Example: Math.sqrt(-1)
• Object: A collection of key-value pairs. Example:

  const obj = {
    name: 'Alex',
    surname: undefined,
  };
  

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Arrays

• Definition: An ordered collection of elements. Example: ['red', 'yellow', 22, false, true]
• Indexing: Starts from 0. Example:

  const personArray = ['mike', 23];
  console.log(personArray[0]); // Output: 'mike'
  

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Objects

• Definition: A collection of properties defined as key-value pairs. Example:

  const car = {
    name: 'Mazda',
    yearOfBuild: 2020,
    color: 'red',
    isNew: true,
    owner: {
      name: 'Oly',
      salary: Infinity,
    },
  };
  

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Variables

• const: Used for values that do not change. Example:

  const ITEMS_TO_FETCH = 10;
  

• let: Used for values that may change. Example:

  let tempToday = '+3';
  

• var: An older way to declare variables (avoid using in modern JavaScript). Example:

  var item = '10';
  item = '12';
  

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Conditional Statements

• if Statement: Executes code blocks based on conditions. Example:

  if (color === 'green') {
    console.log('cars go');
  } else if (color === 'yellow') {
    console.log('cars prepare');
  } else {
    console.log('cars stop');
  }
  

• switch Statement: Useful for multiple conditions. Example:

  switch (color) {
    case 'green':
      console.log('cars go');
      break;
    default:
      console.log('cars stop');
  }
  

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Loops

• For Loop: Iterates through arrays or other collections. Example:

  const names = ['mike', 'john', 'alex'];
  for (let i = 0; i < names.length; i++) {
    console.log(names[i].toUpperCase());
  }
  

• While Loop: Continues while a condition is true. Example:

  let index = 0;
  while (index < names.length) {
    console.log(names[index]);
    index++;
  }
  

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Functions

• Definition: A reusable block of code. Example:

  function updateName(name) {
    return name.toUpperCase();
  }
  console.log(updateName('alex'));
  

• Arrow Functions: A modern shorthand for functions. Example:

  const updateName = (name) => name.toUpperCase();
  

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Practical Examples

• Random Color Generator:

  function createColor() {
    const randomValue = Math.ceil(Math.random() * 100);
    return randomValue < 30 ? 'red' : 'green';
  }
  console.log(createColor());
  

• Save Cart to Database:

  function saveCart(items) {
    // Save to DB
  }
  if (userAddedItemToCart === true) {
    saveCart(updatedCart);
  }
  

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Summary

In this lesson, we explored:

1. Basic data types (string, number, boolean, null, undefined).
2. How to work with objects and arrays.
3. Variables (const, let, var).
4. Control structures (if, switch, loops).
5. Functions (traditional and arrow).

Practice these concepts to strengthen your understanding of JavaScript!