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In addition to variable types, TypeScript allows you to declare the expected types of your function parameters. It'll also infer the types returned by those functions.

Functions

In TypeScript, function parameters may be given type annotations with the same syntax as variable declarations: a colon next to the name. The type annotations ensure that the parameters are of the correct type:

```ts
function greet(name: string) {
  console.log(`Hello, ${name}!`);
}

greet('Katz'); // Prints: Hello, Katz  

greet(1337); // Error: argument '1337' is not assignable to parameter of type 'string'
```

By placing `: string` after the `name` parameter, we’re declaring that `name` is of type `string`. Any invocation of `greet()` should pass a value of type `string` as the first argument, or else an error will be thrown.

Parameters that we do not provide type annotations for are assumed to be of type `any`—the same way variables are.

```ts
function printKeyValue(key: string, value) {
  console.log(`${key}: ${value}`);
}

printKeyValue('Courage', 1337); // Prints: Courage: 1337
printKeyValue('Mood', 'scared'); // Prints: Mood: scared
```

Here, the parameter `value` is an `any` variable: it's compatible with any type. 

Parameter Type Annotations

If a parameter is assigned a default value, TypeScript will infer the variable type to be the same as the default value's type.  (This is similar to how TypeScript infers the type of an initialized variable to be the same as the type of its initial value.)

The following code snippet logs a string to greet a user's name, and defaults to the name `'Anonymous'` if no name is provided. 

```typescript
function greet(name = 'Anonymous') {
  console.log(`Hello, ${name}!`);
}
```

The function `greet()` can receive a `string` or `undefined` as its `name` parameter---if any other type is provided as an argument,  TypeScript will consider that a type error. 

This is a cleaner way of getting the same functionality we had in the previous exercise. There, we used `?` to mark the `name` parameter as optional. But parameters with default values don’t need a `?` after their name, since assigning a default value already implies that they’re optional parameters.


Default Parameters

TypeScript can also infer the types of values returned by functions. It does this by looking at the types of the values after a function's `return` statements.

```ts
function createGreeting(name: string) {
  return `Hello, ${name}!`;
}

const myGreeting = createGreeting('Aisle Nevertell');
```

Here’s how TypeScript can infer `myGreeting` above to be of type `string`:
1. `createGreeting()`’s `return` statement is followed by a  `string` variable, so `createGreeting()` is inferred to return `string`.
2. `createGreeting('Aisle Nevertell')` therefore must result in a value of type `string`.
3. `myGreeting` is initialized to `createGreeting('Aisle Nevertell')`, which is a value with the type `string`.

Cool! Let's see how this can help us fix bugs:

```ts
function ouncesToCups(ounces: number) {
  return `${ounces / 16} cups`;
}

const liquidAmount: number = ouncesToCups(3);
// Type 'string' is not assignable to type 'number'.
```

Here, TypeScript was able to infer that `liquidAmount` was being assigned a value of type `string`, despite it being declared as a variable of type `number`. This correctly results in an error being surfaced.

Inferring Return Types

By now, we've made a pretty convincing case that type annotations are very useful and should always be used unless there's a very good reason not to. They make everything neat and aid in understanding our code.

For these reasons, it is often preferred to use type annotations for functions, even when those functions don't return anything. Example:

```typescript
function logGreeting(name: string){
  console.log(`Hello, ${name}!`)
}
```

The function `logGreeting()` simply logs a greeting to the console. There is no returned value, so we must treat the return type as `void`. A proper type annotation for this function would look like this:

```typescript
function logGreeting(name:string): void{
  console.log(`Hello, ${name}!`)
}
```


Void Return Type

When we declare a function in JavaScript, we often expect it to be invoked with arguments of a certain type.  JavaScript does not share our expectations: its type flexibility often allows functions to be invoked with unexpected argument types. Even when this doesn’t result in thrown errors, there can be negative consequences:

```ts
function printLengthOfText(text) {
  console.log(text.length);
}

printLengthOfText(3); // Prints: undefined
```

JavaScript developers have found error-handling solutions to avoid such undesirable effects, but these techniques can be cumbersome: 

```js
function printLengthOfText(text) {
  if (typeof text !== 'string') {
    throw new Error('Argument is not a string!');
  }

  console.log(text.length);
}

printLengthOfText(3); // Error: Argument is not a string!
```

In the code above, the function first checks the argument type. If it doesn't match the expected type, an error is thrown; otherwise, it continues with its intended operation. 

Before we explore how TypeScript handles this issue, let's practice fixing some JavaScript type-related bugs. These are the kinds of problems that TypeScript helps us diagnose early on, before they become hard to spot.

Introduction

If we’d like to be explicit about what type a function returns, we can add an explicit type annotation after its closing parenthesis. Here, we use the same syntax as other type annotations, a colon followed by the type. TypeScript will produce an error for any return statement in that function that doesn’t return the right type of value.

```ts
function createGreeting(name?: string): string {
  if (name) {
    return `Hello, ${name}!`;
  }

  return undefined;
  // Typescript Error: Type 'undefined' is not assignable to type 'string'.
};
```

We can also explicitly state return types for arrow functions (which were defined in the ES6/ES2015 version of JavaScript). We’ll see the same kinds of error messages for both function types.

```ts
const createArrowGreeting = (name?: string): string => {
  if (name) {
    return `Hello, ${name}!`;
  }

  return undefined;
  // Typescript Error: Type 'undefined' is not assignable to type 'string'.
};
```

Now let's take a look at an example of how explicit return type annotation can be useful when we're working with other people's code. 

Explicit Return Types

We learned all about how to use TypeScript to specify the types of function parameters and return types.  Now we know how to:

* Give type annotations to function parameters.
* Deal with type annotations for optional parameters, which may have default values.
* Understand how TypeScript determines the return type of a function.
* Explicitly specify return types for functions, including for functions that don't return anything.
* Use the above skills to diagnose and fix bugs in our code.

Great job! 



Review

TypeScript recognizes JavaScript comment syntax:
```js
// This is a single line comment 

/*
This is a 
multiline
comment
*/
```
But it's common in TypeScript to see a third comment style: documentation comments. A documentation comment is denoted with the first line `/**` and a final line `*/`. It's common for each line within the comment to start with an asterisk (`*`):
```ts
/**
* This is a documentation comment
*/
```
Documentation comments are especially useful for documenting functions. We place a function's documentation comment in the code directly above the function declaration. We can use special tags within the comment to highlight certain aspects of the function. We can use `@param` to describe each of the function's parameters, and we can use `@returns` to describe what the function returns:
```ts
  /**
   * Returns the sum of two numbers.
   *
   * @param x - The first input number
   * @param y - The second input number
   * @returns The sum of `x` and `y`
   *
   */
  function getSum(x: number, y: number): number {
    return x + y;
  }
}
```
Many text editors will helpfully display documentation comments, for example, when hovering over a function name.  

Documenting Functions

TypeScript normally gives an error if we don’t provide a value for all arguments in a function. This isn’t always desirable; sometimes we’d like to skip providing values.

```typescript
function greet(name: string) {
  console.log(`Hello, ${name || 'Anonymous'}!`);
}

greet('Anders'); // Prints: Hello, Anders!
greet(); // TypeScript Error: Expected 1 arguments, but got 0.
```

When the code snippet above is compiled to JavaScript, the `greet()` function will correctly print `'Hello, Anonymous!'`. That's because when no arguments are passed in, `name` has the falsy value `undefined`, which means that `name || 'Anonymous'` evaluates to `'Anonymous'`.  Since the final code works as intended, we want to prevent TypeScript from throwing errors:

To indicate that a parameter is intentionally optional, we add a `?` after its name. This tells TypeScript that the parameter is allowed to be `undefined` and doesn’t always have to be provided.

```typescript
function greet(name?: string) {
  console.log(`Hello, ${name|| 'Anonymous'}!`);
}

greet(); // Prints: Hello, Anonymous!
```