# ussd-menu-builder

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Easily compose USSD menus in Node.js, compatible with
[Africastalking API](https://africastalking.com) or [Hubtel API](https://developers.hubtel.com/reference#ussd).

## Installation

```
$ npm install ussd-menu-builder
```
or
```
$ yarn add ussd-menu-builder
```

## Features
- Use intuitive states to compose USSD menus
- Makes it easier to build complex nested menus
- Use simple input matching or regular expressions, custom asynchronous
functions to resolve routes from one state to another
- The state-based approach allows you to easily modularize complex menus
in different files

## Quick Example

```javascript
const UssdMenu = require('ussd-menu-builder');
let menu = new UssdMenu();

// Define menu states
menu.startState({
    run: () => {
        // use menu.con() to send response without terminating session      
        menu.con('Welcome. Choose option:' +
            '\n1. Show Balance' +
            '\n2. Buy Airtime');
    },
    // next object links to next state based on user input
    next: {
        '1': 'showBalance',
        '2': 'buyAirtime'
    }
});

menu.state('showBalance', {
    run: () => {
        // fetch balance
        fetchBalance(menu.args.phoneNumber).then(function(bal){
            // use menu.end() to send response and terminate session
            menu.end('Your balance is KES ' + bal);
        });
    }
});

menu.state('buyAirtime', {
    run: () => {
        menu.con('Enter amount:');
    },
    next: {
        // using regex to match user input to next state
        '*\\d+': 'buyAirtime.amount'
    }
});

// nesting states
menu.state('buyAirtime.amount', {
    run: () => {
        // use menu.val to access user input value
        var amount = Number(menu.val);
        buyAirtime(menu.args.phoneNumber, amount).then(function(res){
            menu.end('Airtime bought successfully.');
        });
    }
});

// Registering USSD handler with Express

app.post('/ussd', function(req, res){
    menu.run(req.body, ussdResult => {
        res.send(ussdResult);
    });
});

```

# Guide
## Introduction
The USSD Menu Builder uses a state machine to create a USSD menu. A state
is created for each menu. Each state has a unique name and a set of rules
used to link to other states based on the user input.

### Creating a menu
Before you can create any states, you first need to create an instance of
the menu.

```javascript
const UssdMenu = require('ussd-menu-builder');
const menu = new UssdMenu();
```

### Running the menu
The **```menu.run(args, resultCallback)```** goes through the menu and finds
the appropriate state to run based on the user input.

The **```args```** object should contain the following keys coming from
the  [Africastalking API](https://africastalking.com):

- **`sessionId`**: unique session ID that persists through the entire USSD session,
can be used to store temporary that may be retrieved from different states
during the session
- **`serviceCode`**: the USSD code registered with your serviceCode
- **`phoneNumber`**: the end user's phone Number
- **`text`**: The raw USSD input. It has the following format ```1*2*4*1```:
a string containing the input at each hop, separated by the asterisk symbol (```*```).
This is parsed by the ```UssdMenu``` to find the appropriate state to run at each hop.

After the matched state runs, the resultCallback is called with the response from the state.

**`Note: `** *The menu also returns a promise that can be resolved if you need to do anything with the final response.
for example:*
```javascript
let resp = await menu.run(args) // resultCallback is not necessarry if you intend to run the menu in an async function

```
Here's an example registering a handler with the [express](https://expressjs.com) framework:
```javascript

app.post('/ussd', (req, res) => {
    let args = {
        phoneNumber: req.body.phoneNumber,
        sessionId: req.body.sessionId,
        serviceCode: req.body.serviceCode,
        text: req.body.text
    };
    menu.run(args, resMsg => {
        res.send(resMsg);
    });
})

```

Handling menu.run response:
```javascript

app.post('/ussd', async (req, res) => {
    let args = {
        phoneNumber: req.body.phoneNumber,
        sessionId: req.body.sessionId,
        serviceCode: req.body.serviceCode,
        text: req.body.text
    };
    let resMsg = await menu.run(args);
    res.send(resMsg);
})

```

## Defining states

The **`menu.state(name, options)`** method is used to define states. I takes the name of the state
and an object with the following properites:
- **`run`**: a function that's called when the state is resolved
- **`next` (optional)**: an object that contains rules of how to match the input of this state
to other states. *This is not required for final states*.
- **`defaultNext` (optional)**: the name of the state to default to if the user
input could not be matched by the rules defined in the `next` object.
If not provided, the same state will be used as a fallback i.e. the same menu will
be displayed to the user.

Here's an example:
```javascript
menu.state('stateName', {
    run: function(){
        menu.con('Choose Option' +
            '\n1. Load Account' +
            '\n2. View Catalogue' +
            '\n3. Check Balance'
        );
    },
    next: {
        '1': 'loadAccount',
        '2': 'catalogue',
        '3': 'balance'
    },
    defaultNext: 'invalidOption'
});
```

### The **`run`** function
Each state defines it's own `run` method which is called when that
state is matched. This is where you should place the logic for a given
state.

#### Retrieving user input
Use **```menu.val```** property to access the current user input.

#### Accessing ussd parameters
You can access the ussd parameters through the **```menu.args```** object.
This parameters should come from the API Gateway and are passed to the
```menu.run``` method.

#### Sending the response
You must use either (not both) of the two methods to send
a response to be displayed to the user:
- **`menu.con(msg)`**: Sends the result to be displayed to the user without
terminating the session i.e. the user can reply with further input.
- **`menu.end(msg)`**: Sends the response to be displayed to the user and
requests the session to be terminated i.e. the user cannot provide further
input. **Note:** *This consequently makes the state a final state and therefore the
```next``` object does not need to be defined*

Example:

```javascript
menu.state('thisState', {
    run: function(){
        let value = menu.val;
        let session = getSession(menu.args.sessionId);
        let phone = menu.args.phoneNumber;
        session.set('phone', phone);
        session.set('value', value);
        menu.end('You entered: ' + value);
    }
});
```

### The Start State
This is the first state or first menu to be displayed by the user.
It is created using the **```menu.startState(options)```**. It uses
the reserved name ```'__start__'```.

``` javascript
menu.startState({
    run: function(){
        ...
    }
    next: {
        ...
    }
});
```

***
**Note**: the ```menu.state()``` and ```menu.startState()``` methods return
the same menu object instance for convenience.

```javascript
menu.startState({
    ...
})
.state('state1', {
    ...
})
.state('state2', {
    ...
})
```
***

### Matching States
To link states you use the ```next``` object to map user input to a state name.
You can match input directly by value or with a regular expression.

#### Matching direct values
Simply add the expected string value as a key in the next object.

#### Matching with regular expressions
Begin the key with an asterisk (**```*```**) to indicate that the key should
be treated like a regular expression e.g. ```'*\\[a-zA-Z]+'``` would match
any input containing only lowercase or uppercase letters.

Remember you can use ```menu.val``` in the matched state to retrieve the actual user input.

Example:
```javascript
menu.state('registration', {
    run: function(){
        menu.con('Enter your name');
    },
    next: {
        '*[a-zA-Z]+': 'registration.name'
    }
});

menu.state('registration.name', {
    run: function(){
        let name = menu.val;
        let session = getSession(menu.args.sessionId);
        session.set('name', name);
        menu.con('Enter your email');
    },
    next: {
        '*\\w+@\\w+\\.\\w+': 'registration.email'
    }
});
```

#### Matching with empty rule on Start State
If the start state does not define a ```run``` method, you provide
an empty string as key in ```next``` to redirect to another state.

```javascript
menu.startState({
    next: {
        '': function(){
            if(user){
                return 'userMenu';
            }
            else {
                return 'registerMenu';
            }
        }
    }
});
```
### Linking states
Beside mapping user input directly to a state name, you can map it to
a function with returns a state name, synchronously with a simple
return statement or asynchronously with a callback or a promise.

#### Mapping to a direct state name
```javascript
menu.state('thisState', {
    ...
    next: {
        'input': 'nextState'
    }
})
```

#### Mapping to a synchronous function
```javascript
menu.state('thisState', {
    ...
    next: {
        'input': function(){
            if(test){
                return 'nextState';
            } else {
                return 'otherState';
            }
        }
    }
});
```
#### Mapping to an async function with callback
```javascript
menu.state('thisState', {
    ...
    next: {
        'input': function(callback){
            runAsyncCode(function(err, res){
                if(res){
                    callback('nextState');
                } else {
                    callback('otherState');
                }
            })
        }
    }
});
```

#### Mapping to an async function with promise
```javascript
menu.state('thisState', {
    ...
    next: {
        'input': function(){
            return new Promise((resolve, reject) => {
                resolve('nextState');
            });
        }
    }
});
```

### Jumping to different state
You can jump to a different state from the ```run``` function of one
state using the **```menu.go(stateName)```** method. This effectively
breaks the state chain (subsequent states will not be reachable) 
and is therefore only useful if jumping to a final state.

```javascript
menu.state('thisState', {
    run: function(){
        menu.go('otherState');
    }
});

menu.state('otherState', {
    run: function(){
        menu.end('Thank you!');
    }
});
```

The **```menu.goStart()```** method can be used to jump to the start state
from within another state.

## Nesting states
The library treats a USSD menu like a chain of interlinked states and therefore
has not internal concept of nesting. However you can achieve complex menus
with nested submenus by linking states appropriately. In addition you
could use a naming convention of your choice to make it clearer to see how
states are related. In these examples I used the following convention of 
separating menu levels with a dot.

## Sessions
You can store temporary user data that persists through an entire session.
The library provides a way for you to define your own custom session
handler so you're free to use whatever storage backend or driver you want.
The menu provides an easy interface to set and retrieve session data
within states based on the implementation you provide.

### Configuring handlers

The **`menu.sessionConfig(config)`** method is used to define your session
handler. It accepts an object with the implementations of the following
methods:

- `start` [**`function(sessionId, callback)`**]: used to initialize a new
session, invoked internally by the `menu.run()` method before any state
is called.
- `end` [**`function(sessionId, callback)`**]: used to delete current session,
invoked internally by the `menu.end()` method.
- `set` [**`function(sessionId, key, value, callback)`**]: used to store
a key-value pair in the current session, invoked internally by
`menu.session.set()`.
- `get` [**`function(sessionId, key, callback)`**]: used to retrieve a
value from the current session by key, invoked internally by 
`menu.session.get()`.

#### Example using local memory for storage

```javascript

let sessions = {};

let menu = new UssdMenu();
menu.sessionConfig({
    start: (sessionId, callback){
        // initialize current session if it doesn't exist
        // this is called by menu.run()
        if(!(sessionId in sessions)) sessions[sessionId] = {};
        callback();
    },
    end: (sessionId, callback){
        // clear current session
        // this is called by menu.end()
        delete sessions[sessionId];
        callback();
    },
    set: (sessionId, key, value, callback) => {
        // store key-value pair in current session
        sessions[sessionId][key] = value;
        callback();
    },
    get: (sessionId, key, callback){
        // retrieve value by key in current session
        let value = sessions[sessionId][key];
        callback(null, value);
    }
});

```

***
**Note:** Instead of callbacks, you may also return promises from
those methods:
```javascript
menu.sessionConfig({
    ...
    get: function(sessionId, key){
        return new Promise((resolve, reject) => {
            let value = sessions[sessionId][key];
            resolve(value);
        });
    }
})
```


### Setting and getting data from the current session

And then to add and retrieve data inside states, use the
`menu.session` object:

```javascript

menu.state('someState', {
    run: () => {
        let firstName = menu.val;
        menu.session.set('firstName', firstName)
        .then( () => {
            menu.con('Enter your last name');
        })
    }
    ...
})
...
menu.state('otherState', {
    run: () => {
        menu.session.get('firstName')
        .then( firstName => {
            // do something with the value
            console.log(firstName);
            ...
            menu.con('Next');
        })
    }
})
...
```

***
**Note**: The `menu.session`'s methods also work with callbacks:
```javascript
menu.session.set('key', 'value', (err) => {
    menu.con('...');
});

menu.session.get('key', (err, value) => {
    console.log(value);
    ...
});
```
***

***
**Note**: It's not required to configure a session handler. You can
access your storage driver directly if you prefer. However if you
do configure a handler using the above method then you should provide
implementations for all the 4 methods as shown above..
***

## Errors

`UssdMenu` instances emit an **`error` event** when an error occurs during the
state resolution process (e.g: **"state not found"** or **"run function not defined"**).

```javascript

menu.startState({
    ...
    next: {
        '1': 'nonExistentState'
    }
});

menu.on('error', (err) => {
    // handle errors
    console.log('Error', err);
});


args.text = '1';
menu.run(args);

```

In addition, errors passed to the callback of the session handler's methods or 
rejected by their promises will also trigger the `error` event for convenience
so that you can handle your handle errors in one place.

```javascript

menu.sessionConfig({
    ...
    get: (sessionId, key, callback){
        callback(new Error('error'));
    }
});

menu.on('error', err => {
    // handle errors
    console.log(err);
});

...

menu.state('someState', {
    run: () => {
        menu.session.get('key').then(val => {
            ...
        });
        // you don't have to catch the error here
    }
});

```

## Hubtel Support

As of version 1.1.0, ussd-menu-builder has added support for Hubtel's USSD API by providing the `provider` option when creating the **UssdMenu** object. There are no changes to the way states are defined, and the HTTP request parameters sent by Hubtel are mapped as usual to `menu.args`, and the result of `menu.run` is mapped to the HTTP response object expected by Hubtel (`menu.con` returns a _Type: Respons & `menu.end` returns a Type: Release). The additional HTTP request parameters like Operator, ClientState, and Sequence are not used.

The key difference with Hubtel is that the service only sends the most recent response message, rather than the full route string. The library handles that using the Sessions feature, which requires that a SessionConfig is defined in order to store the session's full route. This is stored in the key `route`, so if you use that key in your application it could cause issues.


### Example

```javascript
menu = new UssdMenu({ provider: 'hubtel' });
// Define Session Config & States normally
menu.sessionConfig({ ... });
menu.state('thisState', {
    run: function(){
        ...
    });
});

app.post('/ussdHubtel', (req, res) => {
    menu.run(req.body, resMsg => {
        // resMsg would return an object like:
        // { "Type": "Response", "Message": "Some Response" }
        res.json(resMsg);
    });
})
```