An API gateway and framework for turning functions into web services
# FunctionScript

![FunctionScript Logo](/images/fs-wordmark.png)

![travis-ci build](https://travis-ci.org/FunctionScript/FunctionScript.svg?branch=master)
![npm version](https://badge.fury.io/js/functionscript.svg)

## An API gateway and framework for turning functions into web services

FunctionScript is a language and specification for turning JavaScript
functions into typed HTTP APIs. It allows JavaScript (Node.js) functions to be
seamlessly exported as HTTP APIs by **defining what the HTTP interface will look
like and how it behaves in the preceding comment block** - including type-safety mechanisms.

FunctionScript arose out of a need to introduce developers with little
programming experience, but familiarity with JavaScript, to full-stack API
development and best practices around defining and connecting HTTP application
interfaces. For this reason, the goals of the language are significantly different than
[TypeScript](https://github.com/microsoft/TypeScript).
**FunctionScript is intended to provide an easy introduction to API development for those of any skill level, while maintaining professional power and flexibility.**

FunctionScript is the primary specification underpinning the [Autocode](https://autocode.com)
platform and its standard library of APIs.

### Quick Example of a FunctionScript API

The following is a real-world excerpt of an API that can be used
to query a Spreadsheet like a Database. The underlying implementation has been
hidden, but the parameters for the API can be seen.

```javascript
/**
* Select Rows from a Spreadsheet by querying it like a Database
* @param {string} spreadsheetId The id of the Spreadsheet.
* @param {string} range The A1 notation of the values to use as a table.
* @param {enum} bounds Specify the ending bounds of the table.
*   ["FIRST_EMPTY_ROW", "FIRST_EMPTY_ROW"]
*   ["FULL_RANGE", "FULL_RANGE"]
* @param {object} where A list of column values to filter by.
* @param {object} limit A limit representing the number of results to return
* @ {number} offset The offset of records to begin at
* @ {number} count The number of records to return, 0 will return all
* @returns {object} selectQueryResult
* @ {string} spreadsheetId
* @ {string} range
* @ {array} rows An array of objects corresponding to row values
*/
module.exports = async (
  spreadsheetId = null,
  range,
  bounds = 'FIRST_EMPTY_ROW',
  where = {},
  limit = {offset: 0, count: 0},
  context
) => {

  /* implementation-specific JavaScript */

  return {/* some data */};

};
```

It generates an API which accepts (and type checks against, following schemas):

- **`spreadsheetId`** A `string`
- **`range`** A `string`
- **`bounds`** An `enum`, can be either `"FIRST_EMPTY_ROW"` or `"FULL_RANGE"`
- **`where`** An `object`
- **`limit`** An `object` that must contain:
   - `limit.offset`, a `number`
   - `limit.count`, a `number`

It will return an `object`:

- **`selectQueryResult`**
   - `selectQueryResult.spreadsheetId` must be a `string`
   - `selectQueryResult.range` must be a `string`
   - `selectQueryResult.rows` must be an `array`

## Background

The impetus for creating FunctionScript is simple: it stems from the initial
vision of [Autocode](https://autocode.com). We believe the modern web is
missing a base primitive - the API. Daily, computer systems and developers around
the planet make trillions of requests to perform specific tasks: process
credit card payments with [Stripe](https://stripe.com), send team messages via
[Slack](https://slack.com), create SMS messages with [Twilio](https://twilio.com).
These requests are made primarily over HTTP: Hypertext Transfer Protocol. However,
little to no "hypertext" is actually sent or received, these use cases have emerged
in an *ad hoc* fashion as a testament to the power of the world wide web. Oftentimes,
API standardization attempts have been presented as band-aids instead of solutions:
requiring developers to jury rig a language, framework, markup language and
hosting provider together just to get a simple "hello world" out the door.

By creating API development standards as part of a **language specification**
instead of a framework, FunctionScript truly treats the web API as a base primitive of
software development instead of an afterthought. This allows teams to be able to
deliver high-quality APIs with the same fidelity as organizations like
Stripe in a fraction of the time without requiring any additional tooling.

# Table of Contents

1. [Introduction](#introduction)
1. [Why FunctionScript?](#why-functionscript)
1. [FunctionScript Examples](#functionscript-examples)
   1. [All Available Types](#all-available-types)
1. [Specification](#specification)
   1. [FunctionScript Resource Definition](#functionscript-resource-definition)
   1. [Context Definition](#context-definition)
   1. [Parameters](#parameters)
      1. [Constraints](#constraints)
      1. [Types](#types)
      1. [Type Conversion](#type-conversion)
      1. [Nullability](#nullability)
   1. [FunctionScript Resource Requests](#functionscript-resource-requests)
      1. [Context](#context)
      1. [Errors](#errors)
         1. [ClientError](#clienterror)
         1. [ParameterError](#parametererror)
            1. [Details: Required](#details-required)
            1. [Details: Invalid](#details-invalid)
         1. [FatalError](#fatalerror)
         1. [RuntimeError](#runtimeerror)
         1. [ValueError](#valueerror)
1. [FunctionScript Server and Gateway: Implementation](#functionscript-server-and-gateway-implementation)
1. [Acknowledgements](#acknowledgements)

# Introduction

To put it simply, FunctionScript defines semantics and rules for turning exported
JavaScript (Node.js) functions into strongly-typed, HTTP-accessible web APIs.
In order to use FunctionScript, you'd set up your own [FunctionScript Gateway](#functionscript-server-and-gateway-implementation) or you would use an existing FunctionScript-compliant service
like [Autocode](https://autocode.com/).

FunctionScript allows you to turn something like this...

```javascript
// hello_world.js

/**
* My hello world function!
*/
module.exports = (name = 'world') => {

  return `hello ${name}`;

};
```

Into a web API that can be called over HTTP like this (GET):

```
https://$user.api.stdlib.com/service@dev/hello_world?name=joe
```

Or like this (POST):

```json
{
  "name": "joe"
}
```

And gives a result like this:

```json
"hello joe"
```

Or, when a type mismatch occurs (like `{"name":10}`):

```json
{
  "error": {
    "type":"ParameterError"
    ...
  }
}
```

## Why FunctionScript?

FunctionScript is intended primarily to provide a scaffold to build and deliver
APIs easily. It works best as a part of the [Autocode](https://autocode.com/)
platform which consumes the FunctionScript API definitions, hosts the code,
generates documentation from the definitions, and automatically handles versioning and
environment management. The reason we've open sourced the language specification
is so that developers have an easier time developing against the highly modular API
ecosystem we've created and can contribute their thoughts and requests.

You can break down the reason for the development of FunctionScript into a few
key points:

- Modern developers and people being introduced to software development for the
  first time are often trying to build web-native scripts. It is *exceedingly*
  difficult to go from "zero to API" in less than a few hours, writing code is
  just the first step of many. We'd like it to be the first and only step.

- No true standards around APIs have ever been built or enforced in a rigorous
  manner across the industry. Primarily, opinions around SOAP, REST and GraphQL
  requests have been built into **frameworks and tools** instead of a
  **language specification**, which has artificially inflated the cognitive
  overhead required to ship functional web-based software.

- Companies like Stripe and Twilio which have built and enforced their own API
  development paradigms internally have unlocked massive developer audiences in
  short timeframes, indicating the power of treating web APIs as a first-class
  citizen of development.

- [Serverless computing](https://en.wikipedia.org/wiki/Serverless_computing),
  specifically the Function-as-a-Service model of web-based computation, has made
  API development significantly more accessible but has not brought us over the
  "last-mile" hump.

- JavaScript, specifically Node.js, is an ideal target for API development
  standardization due to its accessibility (front-end and back-end), growth
  trajectory, and flexibility. Most new developers are introduced to JavaScript
  out of necessity.

- As opposed to something like [TypeScript](https://github.com/microsoft/TypeScript),
  FunctionScript helps newer entrants to software development by extending
  JavaScript with very little overhead. It adds types around *only the HTTP interface*,
  leaving the majority of the language footprint untouched but strengthening the
  "weakest" and least predictable link in the development chain: user input.

With FunctionScript, it's our goal to develop a language specification for
building APIs that automatically provides a number of necessary features without
additional tooling:

- Standardized API Calling Conventions (HTTP)
- Type-Safety Mechanisms at the HTTP -> Code Interface
- Automatically Generated API Documentation

# FunctionScript Examples

We'll be updating this section with examples for you to play with and modify
on your own.

## All Available Types

Here's an example of a hypothetical `createUser.js` function that can be used
to create a user resource. It includes all available type definitions.

```javascript
/**
* @param {integer} id ID of the User
* @param {string} username Name of the user
* @param {number} age Age of the user
* @param {float} communityScore Community score (between 0.00 and 100.00)
* @param {object} metadata Key-value pairs corresponding to additional user data
* @ {string} createdAt Created at ISO-8601 String. Required as part of metadata.
* @ {?string} notes Additional notes. Nullable (not required) as part of object
* @param {array} friendIds List of friend ids
* @ {integer} friendId ID of a user (forces array to have all integer entries)
* @param {buffer} profilePhoto Base64-encoded filedata, read into Node as a Buffer
* @param {enum} userGroup The user group. Can be "USER" (read as 0) or "ADMIN" (read as 9)
*   ["USER", 0]
*   ["ADMIN", 9]
* @param {boolean} overwrite Overwrite current user data, if username matching
* @returns {object.http} successPage API Returns an HTTP object (webpage)
*/
module.exports = async (id = null, username, age, communityScore, metadata, friendsIds = [], profilePhoto, userGroup, overwrite = false) => {

  // NOTE: id, friendIds and overwrite will be OPTIONAL as they have each been
  //       provided a defaultValue

  // Implementation-specific code here

  // API Output
  // NOTE: Note that because "object.http" was specified, this MUST follow the
  //       object.http schema: headers, statusCode, body

  return {
    headers: {'Content-Type': 'text/html'},
    statusCode: 200,
    body: Buffer.from('Here is a success message!')
  };

};
```

# Specification

## FunctionScript Resource Definition

A FunctionScript definition is a JSON output, traditionally saved as a
`definition.json` file, generated from a JavaScript file,
that respects the following format.

Given a function like this (filename `my_function.js`):

```javascript
// my_function.js

/**
* This is my function, it likes the greek alphabet
* @param {String} alpha Some letters, I guess
* @param {Number} beta And a number
* @param {Boolean} gamma True or false?
* @returns {Object} some value
*/
module.exports = async (alpha, beta = 2, gamma, context) => {
  /* your code */
};
```

The FunctionScript parser will generate a `definition.json` file that looks
like the following:

```json
{
  "name": "my_function",
  "format": {
    "language": "nodejs",
    "async": true
  },
  "description": "This is my function, it likes the greek alphabet",
  "bg": {
    "mode": "info",
    "value": ""
  },
  "context": null,
  "params": [
    {
      "name": "alpha",
      "type": "string",
      "description": "Some letters, I guess"
    },
    {
      "name": "beta",
      "type": "number",
      "defaultValue": 2,
      "description": "And a number"
    },
    {
      "name": "gamma",
      "type": "boolean",
      "description": "True or false?"
    }
  ],
  "returns": {
    "type": "object",
    "description": "some value"
  }
}
```

A definition must implement the following fields;

| Field | Definition |
| ----- | ---------- |
| name | A user-readable function name (used to execute the function), must match `/[A-Z][A-Z0-9_]*/i` |
| format | An object requiring a `language` field, along with any implementation details |
| description | A brief description of what the function does, can be empty (`""`) |
| bg | An object containing "mode" and "value" parameters specifying the behavior of function responses when executed in the background |
| params | An array of `NamedParameter`s, representing function arguments
| returns | A `Parameter` without a `defaultValue` representing function return value |

## Context Definition

If the function does not access execution context details, this should always
be null. If it is an object, it indicates that the function *does* access
context details (i.e. `remoteAddress`, http headers, etc. - see [Context](#context)).

This object **does not have to be empty**, it can contain vendor-specific
details; for example `"context": {"user": ["id", "email"]}` may indicate
that the execution context specifically accesses authenticated user id and email
addresses.

## Parameters

Parameters have the following format;

| Field | Required | Definition |
| ----- | -------- | ---------- |
| name | NamedParameter Only | The name of the Parameter, must match `/[A-Z][A-Z0-9_]*/i` |
| type | yes | A string representing a valid FunctionScript type |
| description | yes | A short description of the parameter, can be empty string (`""`) |
| defaultValue | no | Must match the specified type. **If type is not provided, this parameter is required** |

### Types

As FunctionScript interfaces with "userland" (user input),
a strongly typed signature is enforced for all inbound parameters. The following
is a list of supported FunctionScript types.

| Type | Definition | Example Input Values (JSON) |
| ---- | ---------- | -------------- |
| boolean | True or False | `true` or `false` |
| string | Basic text or character strings | `"hello"`, `"GOODBYE!"` |
| number | Any double-precision [Floating Point](https://en.wikipedia.org/wiki/IEEE_floating_point) value | `2e+100`, `1.02`, `-5` |
| float | Alias for `number` | `2e+100`, `1.02`, `-5` |
| integer | Subset of `number`, integers between `-2^53 + 1` and `+2^53 - 1` (inclusive) | `0`, `-5`, `2000` |
| object | Any JSON-serializable Object | `{}`, `{"a":true}`, `{"hello":["world"]}` |
| object.http | An object representing an HTTP Response. Accepts `headers`, `body` and `statusCode` keys | `{"body": "Hello World"}`, `{"statusCode": 404, "body": "not found"}`, `{"headers": {"Content-Type": "image/png"}, "body": Buffer.from(...)}` |
| array | Any JSON-serializable Array | `[]`, `[1, 2, 3]`, `[{"a":true}, null, 5]` |
| buffer | Raw binary octet (byte) data representing a file | `{"_bytes": [8, 255]}` or `{"_base64": "d2h5IGRpZCB5b3UgcGFyc2UgdGhpcz8/"}` |
| any | Any value mentioned above | `5`, `"hello"`, `[]` |
| enum | An enumeration that maps input strings to values of your choosing | `"STRING_OF_YOUR_CHOICE"` |

### Type Conversion

The `buffer` type will automatically be converted from any `object` with a
**single key-value pair matching the footprints** `{"_bytes": []}` or `{"_base64": ""}`.

Otherwise, parameters provided to a function are expected to match their
defined types. Requests made over HTTP via query parameters or POST data
with type `application/x-www-form-urlencoded` will be automatically
converted from strings to their respective expected types, when possible
(see [FunctionScript Resource Requests](#functionscript-resource-requests) below):

| Type | Conversion Rule |
| ---- | --------------- |
| boolean | `"t"` and `"true"` become `true`, `"f"` and `"false"` become `false`, otherwise **do not convert** |
| string | No conversion |
| number | Determine float value, if NaN **do not convert**, otherwise convert |
| float | Determine float value, if NaN **do not convert**, otherwise convert |
| integer | Determine float value, if NaN **do not convert**, may fail integer type check if not in range |
| object | Parse as JSON, if invalid **do not convert**, object may fail type check (array, buffer) |
| object.http | Parse as JSON, if invalid **do not convert**, object may fail type check (array, buffer) |
| array | Parse as JSON, if invalid **do not convert**, object may fail type check (object, buffer) |
| buffer | Parse as JSON, if invalid **do not convert**, object may fail type check (object, array) |
| any | No conversion |
| enum | Read input as string |

### Nullability

All types are potentially nullable, an nullability can be defined in two ways:

**(1)** by setting `"defaultValue": null` in the `NamedParameter` definition.

```
/**
* @param {string} nullableString
*/
module.exports = (nullableString = null) => {
  return `Test ${nullableString}`;
}
```

**(2)** By prepending a `?` before the type name in the comment definition, i.e.:

```
/**
* @param {?string} nullableString
*/
module.exports = (nullableString) => {
  return `Test ${nullableString}`;
}
```

**NOTE:** That the difference between this two behaviors is that the latter
will mean `nullableString` is both `required` AND `nullable`, whereas the former
means `nullableString` has a `defaultValue` (is optional).

### Setting HTTP headers

The `object.http` type should be used to generate HTTP responses that are intended
to return more complex data than simple JSON responses.

You can provide `headers`, `statusCode` and `body` in an `object.http` response.

For example, to return an image that's of type `image/png`...

```javascript
/**
* Retrieves an image
* @param {string} imageName The name of the image
* @returns {object.http} image The result
*/
module.exports = (imageName) => {

  // fetch image, returns a buffer
  let png = imageName === 'cat' ?
    fs.readFileSync(`/images/kitty.png`) :
    fs.readFileSync(`/images/no-image.png`);

  // Forces image/png over HTTP requests, default
  //  for buffer would otherwise be application/octet-stream
  return {
    headers: {'Content-Type': 'image/png'},
    statusCode: 200,
    body: png
  };

};
```

## FunctionScript Resource Requests

FunctionScript requests *must* complete the following steps;

1. Ensure the **Resource Definition** is valid and compliant, either on storage
    or accession.
1. Performs a handshake (i.e. HTTP) with initial request details
1. Accept an `Array`, `Object` or a string of URLencoded variables
1. If over HTTP and query parameters present, query parameters used as
   URL encoded variables
1. If over HTTP POST and query parameters present, reject requests that try to
   specify a POST body as well with a `ClientError`
1. If over HTTP POST, requests **must** include a `Content-Type` header or
   a `ClientError` is immediately returned
1. If over HTTP POST, `Content-Type` **must** be `application/json` for `Array`
   or `Object` data, or `application/x-www-form-urlencoded` for string data or
   a `ClientError` is immediately returned
1. If `application/x-www-form-urlencoded` values are provided (either via POST
   body or query parameters), convert types based on [Type Conversion](#type-conversion)
   and knowledge of the function definition and create an `Object`
1. If `Array`: Parameters will be checked for type consistency in the order of
   the definition `params`
1. If `Object`: Parameters will be checked for type consistency based on names
   of the definition `params`
1. If any inconsistencies are found, cease execution and immediately return a
   `ParameterError`
1. If a parameter has no defaultValue specified and is not provided, immediately
   return a `ParameterError`
1. Try to execute the function, if the function fails to parse or is not valid,
   immediately return a `FatalError`
1. If a function hits a specified timeout (execution time limit), immediately
   return a `FatalError`
1. If a function returns an error (via callback) or one is thrown and not caught,
   immediately return a `RuntimeError`
1. If function returns inconsistent response (does not match `returns` type),
   immediately return a `ValueError`
1. If no errors are encountered, return the value to the client
1. If over HTTP and `content-type` is not being overloaded (i.e. developer
   specified through a vendor-specific mechanism), return `buffer` type data as
   `application/octet-stream` and any other values as `application/json`.

### Context

Every function intended to be consumed via FunctionScript has the option to specify
an *optional* magic `context` parameter that receives vendor-specific
information about the function execution context - for example, if consumed over
HTTP, header details. FunctionScript definitions must specify whether or not they
consume a `context` object. Context objects are extensible but **MUST** contain
the following fields;

| Field | Definition |
| ----- | ---------- |
| params | An `object` mapping called parameter names to their values |
| http | `null` if not accessed via http, otherwise an `object` |
| http.headers | If accessed via HTTP, an `object` containing header values |

### Errors

Errors returned by FunctionScript-compliant services must follow the following JSON
format:

```json
{
  "error": {
    "type": "ClientError",
    "message": "You know nothing, Jon Snow",
    "details": {}
  }
}
```

`details` is an optional object that can provide additional Parameter details.
Valid Error types are:

- `ClientError`
- `ParameterError`
- `FatalError`
- `RuntimeError`
- `ValueError`

#### ClientError

`ClientError`s are returned as a result of bad or malformed client data,
  including lack of authorization or a missing function (not found). If over
  HTTP, they **must** returns status codes in the range of `4xx`.

#### ParameterError

`ParameterError`s are a result of Parameters not passing type-safety checks,
  and **must** return status code `400` if over HTTP.

Parameter Errors **must** have the following format;

```json
{
  "error": {
    "type": "ParameterError",
    "message": "ParameterError",
    "details": {...}
  }
}
```

`"details"` should be an object mapping parameter names to their respective
validation (type-checking) errors. Currently, this specification defines
two classifications of a ParameterError for a parameter; *required* and
*invalid*. The format of `"details": {}` should follow this format;

##### Details: Required

```json
{
  "param_name": {
    "message": "((descriptive message stating parameter is required))",
    "required": true
  }
}
```

##### Details: Invalid

```json
{
  "param_name": {
    "message": "((descriptive message stating parameter is invalid))",
    "invalid": true,
    "expected": {
      "type": "number"
    },
    "actual": {
      "type": "string",
      "value": "hello world"
    }
  }
}
```

#### FatalError

`FatalError`s are a result of function mismanagement - either your function
  could not be loaded, executed, or it timed out. These **must** return status
  code `500` if over HTTP.

#### RuntimeError

`RuntimeError`s are a result of uncaught exceptions in your code as it runs,
  including errors you explicitly choose to throw (or send to clients via a
  callback, for example). These **must** return status code `403` if over
  HTTP.

#### ValueError

`ValueError`s are a result of your function returning an unexpected value
  based on FunctionScript type-safety mechanisms. These **must** return status code
  `502` if over HTTP.

`ValueError` looks like an *invalid* ParameterError, where the `details`
Object only ever contains a single key called `"returns"`. These are encountered
due to implementation issues on the part of the function developer.

```json
{
  "error": {
    "type": "ValueError",
    "message": "ValueError",
    "details": {
      "returns": {
        "message": "((descriptive message stating return value is invalid))",
        "invalid": true,
        "expected": {
          "type": "boolean"
        },
        "actual": {
          "type": "number",
          "value": 2017
        }
      }
    }
  }
}
```

# FunctionScript Server and Gateway: Implementation

A fully-compliant FunctionScript gateway (that just uses local function resources)
is available with this package, simply clone it and run `npm test` or look
at the `/tests` folder for more information.

The FunctionScript specification is used as the platform specification
for [Autocode](https://autocode.com), and is available for local use with the
[Autocode CLI](https://github.com/acode/cli) which relies on this
repository as a dependency.

# Acknowledgements

FunctionScript is the result of years of concerted effort working to make API
development easier. It would not be possible without the personal and financial
commitments of some very amazing people and companies. We'd like to thank our
customers, investors, supporters, friends and family.

### Core Contributors

- [**Keith Horwood**](https://twitter.com/keithwhor)
- [**Jacob Lee**](https://twitter.com/hacubu)
- [**Steve Meyer**](https://twitter.com/notoriaga)
- [**Yusuf Musleh**](https://twitter.com/yusuf-musleh)

# Notes

The software contained within this repository has been developed and is
copyrighted by the [Autocode](https://autocode.com) team (Polybit Inc.)
and is MIT licensed.
