# Interfaces in TypeScript

[Handbook](https://www.typescriptlang.org/docs/handbook/interfaces.html)

## Overview

Interface in TypeScripts are used to create a _blueprint_ or an abstraction of a class.

An interface can contains properties (_mandatory_, _optional_ or _readonly_)

```typescript
interface Example {
  mandatory: string;
  optional?: string;
  readonly ro: string;
}
```

And methods

```typescript
interface Example {
  mandatory: string;
  optional?: string;

  concat(sep: string): string;
}
```

To use the interface, a class should **implement** it,

```typescript
class Impl implements Example {
  mandatory: string;
  optional?: string;

  constructor(m: string, o: string) {
    this.mandatory = m;
    this.optional = o;
  }

  contact(sep: string): string {
    return `${this.mandatory}${sep}${this.optional}`;
  }
}
```

## The Promise case

What about a promise based method ? Do not forget what async/await does. It wraps all the things in `Promise<T>`.

In order to add promise to your interface, just specify a Promise as return type.

```typescript
interface Example {
  mandatory: string;
  optional?: string;

  promize(arg: string): Promise<string>;
}
```