# Function signatures

Let's look at how we can add explicit types to our function signatures.

## Parameter types

To add types to your parameter list, simply add annotate the parameter with `: {type}`.

For example:

```typescript
function add(x: number, y: number) {
  return x + y;
}
```

Here TypeScript is actually able to infer that the return type is `number`. Smart! However, we want to be explicit about the return type, so let's annotate it.

## Return type

To add a return type, simply add `: {type}` after the `()` of your parameter declarations.

For example:

```typescript
function add(x: number, y: number): number {
  return x + y;
}
```

Now it's very clear that this function returns a `number` and only a `number`.

If you don't have an explicit return, use [`void`](/typescript/beginner/functions/void-type.md).

## Optional parameters

TypeScript supports optional parameters.

To mark a parameter as optional, simply append a `?` to the parameter's name.

For example:

```typescript
function foo(bar: number, baz?: string): void {
  // ...
}

foo(123); // OK
foo(123, 'hello'); // OK
foo(123, 123); // Error: 2nd parameter not of type 'string'.
foo(123, 'hello', 'extra param'); // Error: expected 1-2 arguments, but got 3.
```

## Default parameters

Additionally, you can provide a default value (using `= someValue` after the parameter declaration) which will get assigned if the caller doesn't provide that argument.

For example:

```typescript
function logGreeting(greeting: string = 'Hello!'): void {
  console.log(greeting);
}

logGreeting(); // logs "Hello!"
logGreeting('Oi como vai!'); // logs "Oi como vai!"
```


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