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# Basic Types in TypeScript

TypeScript provides several basic types that are essential for building robust applications. These include `number`, `string`, `boolean`, `null`, and `undefined`.

Let's create some simple applications to understand how to use these types effectively.

**Problem 1 - Hello World**

**Thing to Learn**: How to give types to function arguments.

**Task**: Write a function that greets a user given their first name.

* **Argument**: `firstName` (string)
* **Logs**: "Hello {firstName}"
* **Return**: None

**Solution**:

```typescript
function greetUser(firstName: string): void {
    console.log(`Hello ${firstName}`);
}

// Example usage:
greetUser('Alice');
```

**Problem 2 - Sum Function**

**Thing to Learn**: How to assign a return type to a function.

**Task**: Write a function that calculates the sum of two numbers.

* **Arguments**: `a` (number), `b` (number)
* **Returns**: Sum of `a` and `b` (number)

**Solution**:

```typescript
function sum(a: number, b: number): number {
    return a + b;
}

// Example usage:
console.log(sum(5, 7));  // Output: 12
```

**Problem 3 - Return True or False Based on Age**

**Thing to Learn**: Type inference.

**Task**: Write a function that returns true if a user is 18 or older.

* **Function Name**: `isLegal`
* **Argument**: `age` (number)
* **Returns**: `true` if `age` is 18 or older, otherwise `false` (boolean)

**Solution**:

```typescript
function isLegal(age: number): boolean {
    return age >= 18;
}

// Example usage:
console.log(isLegal(20));  // Output: true
console.log(isLegal(17));  // Output: false
```

**Problem 4 - Run a Function After 1 Second**

**Thing to Learn**: How to handle functions as arguments.

**Task**: Create a function that takes another function as input and runs it after 1 second.

* **Function Name**: `runAfterOneSecond`
* **Argument**: `fn` (function)
* **Returns**: None

**Solution**:

```typescript
function runAfterOneSecond(fn: () => void): void {
    setTimeout(fn, 1000);
}

// Example usage:
runAfterOneSecond(() => {
    console.log('This runs after 1 second');
});
```

These examples illustrate the use of TypeScript's basic types and provide a foundation for understanding how to work with typed arguments, return types, and type inference in functions.