# JavaScript Cheat Sheet

## Table of Contents

1. [Variables](#variables)
2. [Data Types](#data-types)
3. [Operators](#operators)
4. [Control Structures](#control-structures)
5. [Functions](#functions)
6. [Arrays](#arrays)
7. [Strings](#strings)
8. [Objects](#objects)
9. [DOM Manipulation](#dom-manipulation)
10. [Asynchronous JavaScript](#asynchronous-javascript)
11. [Error Handling](#error-handling)

## Variables

### Declaration and Assignment
```javascript
let variableName = value;
```

### Variable Types
- String: `let stringVariable = "Hello";`
- Number: `let numberVariable = 42;`
- Boolean: `let boolVariable = true;`
- Undefined: `let undefinedVariable;`
- Null: `let nullVariable = null;`

## Data Types

### Primitive Types
- String
- Number
- Boolean
- Undefined
- Null

### Reference Types
- Object
- Array
- Function

## Operators

### Arithmetic Operators
```javascript
+, -, *, /, %, ++, --
```

### Comparison Operators
```javascript
==, ===, !=, !==, <, >, <=, >=
```

### Logical Operators
```javascript
&& (and), || (or), ! (not)
```

### Assignment Operators
```javascript
=, +=, -=, *=, /=, %=
```

## Control Structures

### If Statement
```javascript
if (condition) {
    // code to be executed if the condition is true
} else if (anotherCondition) {
    // code to be executed if the previous condition was false and this one is true
} else {
    // code to be executed if all conditions are false
}
```

### Switch Statement
```javascript
switch (variable) {
    case value1:
        // code to be executed if variable equals value1
        break;
    case value2:
        // code to be executed if variable equals value2
        break;
    default:
        // code to be executed if variable doesn't match any case
}
```

### Loops
#### For Loop
```javascript
for (let i = 0; i < 5; i++) {
    // code to be executed in each iteration
}
```

#### While Loop
```javascript
while (condition) {
    // code to be executed as long as the condition is true
}
```

#### Do-While Loop
```javascript
do {
    // code to be executed at least once, then repeated as long as the condition is true
} while (condition);
```

## Functions

### Function Declaration
```javascript
function functionName(param1, param2) {
    // code to be executed
    return result; // optional
}
```

### Arrow Functions
```javascript
const add = (a, b) => a + b;
```

### Function Call
```javascript
let result = functionName(arg1, arg2);
```

## Arrays

### Array Declaration
```javascript
let arrayName = [value1, value2, value3];
```

### Array Methods
```javascript
arrayName.push(newValue); // Add to the end
arrayName.pop(); // Remove from the end
arrayName.unshift(newValue); // Add to the beginning
arrayName.shift(); // Remove from the beginning
```

## Strings

### Template Literals
```javascript
let name = "World";
let greeting = `Hello, ${name}!`;
```

### String Methods
```javascript
let str = "Hello, World!";
str.length; // Length of the string
str.indexOf("World"); // Index of the first occurrence of "World"
str.slice(start, end); // Extract a portion of the string
```

## Objects

### Object Declaration
```javascript
let person = {
    name: "John",
    age: 30,
    isStudent: false
};
```

### Object Methods
```javascript
person.sayHello = function() {
    console.log(`Hello, my name is ${this.name}.`);
};
```

## DOM Manipulation

### Selecting Elements
```javascript
let elementById = document.getElementById("elementId");
let elementByClass = document.getElementsByClassName("className");
let elementByTag = document.getElementsByTagName("tagName");
```

### Modifying Elements
```javascript
elementById.innerHTML = "New content";
elementById.style.color = "red";
```

## Asynchronous JavaScript

### Callbacks
```javascript
function fetchData(callback) {
    // asynchronous operation
    setTimeout(() => {
        let data = "This is the data.";
        callback(data);
    }, 1000);
}

fetchData((result) => {
    console.log(result);
});
```

### Promises
```javascript
function fetchData() {
    return new Promise((resolve, reject) => {
        // asynchronous operation
        setTimeout(() => {
            let data = "This is the data.";
            resolve(data);
            // reject("Error occurred"); // Uncomment to simulate an error
        }, 1000);
    });
}

fetchData()
    .then(result => console.log(result))
    .catch(error => console.error(error));
```

## Error Handling

### Try-Catch Block
```javascript
try {
    // code that may throw an error
} catch (error) {
    // handle the error
    console.error("Error: " + error.message);
}
```

```