permissions is a small and simple Nodejs and Express utility for controlling user actions permissions based on a predefined set of rules.

Examples:

Example #1:

You can use it as an Express middleware:

app.get(
  '/users/:id',
  permissionsMw((req) => req.params.id === req.$loggedInUser.id, { denyMsg: 'You can only read your own data' }),
  (req, res, next) => {
    // ... rest of the code
  }
)

Example #2:

or you can use it as a regular JavaScript function

app.get('/users/:id', async (req, res, next) => {
  await permissions(() => req.params.id === req.$loggedInUser.id, { denyMsg: 'You can only read your own data' })
})

API:

The API is pretty simple and straightforward.

permissionsMw(permission: boolean | function, options: object | function): Express middleware

parameters:

• permission: boolean or function.
  • If true was provided, it indicates grant permission, which calls next().
  • If false was provided, it indicates permission denied , which marks the operation as access denied.
  • If undefined or nothing was provided, the defaultBehaviour option determines what, (which is by default to deny access).
  • If a function was provided, it will be called with the req object as the first argument. Your function can return true, false or undefined, and the same behavior happens.
• options: object | function, options to configure how permissionsMw() works.
  • denyMsg: any, the message to include in the error in case the access denied happen, (default: 'You don't have the permission to perform this action').
  • denyStatusCode: number, the status code to include in the error in case the access denied happen, (default 401).
  • handlePermissionErr: boolean, by default, if access denied happen, permissionsMw sends the response to the client with a meaningful error, in case you wanted to handle that error yourself, change this option to false, and it will call next() with the error (default true).
  • defaultBehaviour: 'deny' | 'access', default behavior if not true or false were the resolved value of the permission argument (ex, permissionsMw(undefined)) (default 'deny').

permissions(permission: boolean | function, options: object): Promise

parameters:

• permission: boolean or function.
  • If true was provided, it indicates grant permission, which calls next().
  • If false was provided, it indicates permission denied , which marks the operation as access denied.
  • If undefined or nothing was provided, the defaultBehaviour option determines what, (which is by default to deny access).
  • If a function was provided, it will be called with the req object as the first argument. Your function can return true, false or undefined, and the same behavior happens.
• options: object, options to configure how permissionsMw() works.
  • denyMsg: any, the message to include in the error in case the access denied happen, (default: 'You don't have the permission to perform this action').
  • denyStatusCode: number, the status code to include in the error in case the access denied happen, (default 401).
  • defaultBehaviour: 'deny' | 'access', default behavior if not true or false were the resolved value of the permission argument (ex, permissionsMw(undefined)) (default 'deny').

PermissionsMw and Permissions classes

These are the constructor classes for permissions() and permissionsMw() methods, you can use these classes to generate a set of methods with pre-defined options, and the best way to describe this process is by showing you an example:

const expressAsyncHandler = require('express-async-handler')
const { Permissions, PermissionsMw } = require('permissions')

const customPermissions = new Permissions({
  notFoundMsg: 'Ľutujeme, ale požadovaný záznam sa nenašiel!',
}).method
const customPermissionsMw = new PermissionsMw({
  notFoundMsg: 'Entschuldigung, aber der angeforderte Datensatz wurde nicht gefunden!',
}).method

app.get('/users', customPermissionsMw(false))
app.patch(
  '/users',
  expressAsyncHandler(async (req, res, next) => {
    await customPermissions(false)
  })
)

• All the constructors accept only one argument as an object, the options parameter.
• You can find the available options for each class in the API section above.

Error handling:

The only error that permissions trigger is the permissionsDenyError, this error includes the following properties:

• name: 'permissionsDenyError',
• statusCode: the status code chosen when the deny access thing happen, (default 401)
• message: (default: "You don't have the permission to perform this action")
• stack: the error call stack.

To handle the error when it gets thrown, you can wrap the permissions calls with something like try/catch blocks, or use await-to, or you may do other logic, However you're must likely not going to handle the error through these ways, most likely you'll be having an express Middleware wrapper such as express-async-handler which calls next() in case any error was thrown in the code it wraps, ex:

app.get(
  expressAsyncHandler(async (req, res, next) => {
    await permissions(() => {
      /* some code */
      if (true) return false
    })
    // the rest of your code
  })
)

To handle the error which gets called by next, you should have an express error handling middleware which handles that, ex:

app.get(
  '/users/:id',
  permissionsMw((req) => false)
)

app.use((err, req, res, next) => {
  if (err.name === 'permissionsDenyError') {
    res.status(err.statusCode).json({
      status: 'fail',
      message: err.message,
    })
  }
})

FAQ:

Q: Can I make the access deny message an object or an array instead of a string?

• Yes you can, if you still need more flexibility, check out the handlePermissionErr option.