# JavaScript Skills

## Variables & Types

### Variables
```javascript
// Variables
let name = "JavaScript";
const PI = 3.14159;
var oldStyle = "deprecated";

// Types
typeof name; // "string"
typeof 42;   // "number"
typeof true;  // "boolean"
```

## Functions

### Regular Function
```javascript
function greet(name) {
    return `Hello, ${name}!`;
}
```

### Arrow Function
```javascript
const greet = (name) => `Hello, ${name}!`;

// Multi-line
const multiply = (a, b) => {
    return a * b;
};
```

### Callback
```javascript
const processData = (data, callback) => {
    const result = doSomething(data);
    callback(result);
};
```

## Arrays

### Array Methods
```javascript
const arr = [1, 2, 3, 4, 5];

// map - transform
arr.map(x => x * 2); // [2, 4, 6, 8, 10]

// filter - select
arr.filter(x => x > 2); // [3, 4, 5]

// reduce - accumulate
arr.reduce((sum, x) => sum + x, 0); // 15

// find - search
arr.find(x => x === 3); // 3

// forEach - iterate
arr.forEach(x => console.log(x));
```

## Objects & Destructuring

### Object
```javascript
const person = {
    name: "John",
    age: 30,
    greet() {
        return `Hello, I'm ${this.name}`;
    }
};

// Access
person.name;
person['age'];
```

### Destructuring
```javascript
const { name, age } = person;
const [first, second] = arr;
```

## Async/Await & Promise

### Promise
```javascript
const fetchData = () => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve("data");
        }, 1000);
    });
};
```

### Async/Await
```javascript
const getData = async () => {
    try {
        const response = await fetch(url);
        const data = await response.json();
        return data;
    } catch (error) {
        console.error(error);
    }
};
```

### Promise.all
```javascript
const [result1, result2] = await Promise.all([
    fetch(url1),
    fetch(url2)
]);
```

## Common Patterns

### Optional Chaining
```javascript
const value = obj?.nested?.property ?? "default";
```

### Nullish Coalescing
```javascript
const value = input ?? "default value";
```

### Spread Operator
```javascript
const merged = { ...obj1, ...obj2 };
const combined = [...arr1, ...arr2];
```