# Fundamentals of HarmonyOS Development

## Data Storage

HarmonyOS applications offer multiple data storage options:

- *Preferences*: Lightweight key-value storage for simple data - *SQLite*: Relational database for structured data storage - *File Storage*: Direct file system access for custom data storage

## Arrays

Arrays are ordered collections of data accessible by index:

```javascript let numbers = [1, 2, 3, 4, 5]; console.log(numbers[0]); // Outputs 1 ```

## Functions

### Regular Functions Encapsulate reusable blocks of code: ```javascript function greet(name) { return "Hello, " + name; } console.log(greet("Alice")); // Outputs "Hello, Alice" ```

### Arrow Functions Concise function syntax: ```javascript const greet = (name) => "Hello, " + name; console.log(greet("Bob")); // Outputs "Hello, Bob" ```

## Interfaces

Define object structure contracts: ```typescript interface Person { name: string; age: number; greet(): string; }

class Employee implements Person { name: string; age: number;

     constructor(name: string, age: number) {
         this.name = name;
         this.age = age;
     }
     
     greet(): string {
         return "Hello, I am " + this.name;
     }
 }
 ```

## Objects and Methods

Collections of properties and methods: ```javascript let person = { name: "Charlie", age: 30, greet: function() { return "Hello, " + this.name; } }; console.log(person.greet()); // Outputs "Hello, Charlie" ```

## Union Types

Values that can be one of several types: ```typescript let age: number | string; age = 25; // Valid age = "twenty-five"; // Also valid ```

## Enums

Named constant collections: ```typescript enum Direction { Up, Down, Left, Right } let direction: Direction = Direction.Up; // Uses enum value ```

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## UI Development Concepts and Layout

### Component Properties and Methods - *Properties*: Configure appearance and behavior - *Methods*: Perform component-specific operations

### Text Styling - *Text Color*: Set via `text-color` property - *Text Overflow*: Control with `text-overflow` (e.g., `ellipsis` displays truncation dots)

### Image Component ```html <image src="path/to/image.png" width="100" height="100"></image> ```

### Input Field ```html <input type="text" placeholder="Enter text here"></input> ```

### SVG Icons Embed directly or reference SVG files: ```html <svg width="100" height="100"> <circle cx="50" cy="50" r="40" stroke="black" stroke-width="3" fill="red" /> </svg> ```

### Layout Elements Organize UI component positioning and sizing

#### Margin ```css .container { margin: 10px; /* Uniform margin / margin: 5px 10px; / Vertical | Horizontal / margin: 5px 10px 15px; / Top | Horizontal | Bottom / margin: 5px 10px 15px 20px; / Top | Right | Bottom | Left / } ```

#### Borders ```css .box { border: 1px solid black; / Width | Style | Color / border-radius: 10px; / Rounded corners / } ```

#### Custom Shapes ```css .special-shape { border-radius: 20px 10px 30px 5px; / Custom rounded corners / } ```

### Backgrounds ```css .background { background-color: #f0f0f0; / Solid color / background-image: url('path/to/image.png'); / Background image / background-position: center; / Image positioning / background-size: cover; / Image sizing / background-repeat: no-repeat; / Prevent tiling / } ```

### Linear Layout Alignment ```css .linear-layout { display: flex; justify-content: center; / Main axis alignment / align-items: center; / Cross axis alignment / flex-direction: row; / Layout direction (row/column) */ } ```