# For Typescript Users

All mitter.io web/javacsript SDKs are written in typescript and as such are bundled with typings automatically. This section details on how to optimally use the mitter.io libraries if you are using typescript.

### Type-matching functions

If you are using type-matching functions to get the type of a payload for example, the functions are implemented as predicates, so typescript will automatically cast it within the branch. For example:

```javascript
mitter.subscribeToPaylod(payload => {
    if (payload['@type'] === 'NewMessagePayload') {
        // The next line will throw an error, since the type of
        // payload is MessagingPipelinePayload
        console.log('New message', payload.message.textPayload)
    } else {
        // The next will not throw an error, since the callback
        // argument is already typed to MessagingPipelinePayload
        console.log('New payload', payload.globalPipelinePayloadId)
    }
})

```

Instead, if you were to use the bundled type-matching functions:

```javascript
mitter.subscribeToPayload(payload => {
    if (isNewMessagingPayload(payload)) {
        // The next line is OK, since the isNewMessagingPayload is a type
        // predicate, which tells the compiler that if true, then the
        // argument (payload) was of type NewMessagingPayload
        console.log('New message', payload.message.textPayload)
    }
})
```

### API calls using clients

All bundled clients are typed and will throw an error when using incorrect request/response values

```javascript
mitter.clients().channels().newChannel({
    // ERROR. Missing property 'defaultRuleSet'
    channelId: 'my-new-channel'
})

mitter.clients().channels().newChannel(new Channel(...))
    // ERROR. No property `wrongProperty` on type Channel
    .then(x => console.log('New channel', channel.wrongProperty)
```

> **NOTE** The above does not apply if you are using fetch and/or axios to make API calls. The internal clients that are used with [restyped](https://github.com/rawrmaan/restyped) are exposed in case you wish to use them with your own axios clients. Do refer to the ts-docs bundled with `@mitter-io/core` on how to access these objects.


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