Say you have a POST endpoint that accepts the name, email and age parameters:

const express = require('express')
const app = express()

app.use(express.json())

app.post('/form', (req, res) => {
  const name  = req.body.name
  const email = req.body.email
  const age   = req.body.age
})

How do you server-side validate those results to make sure

  • name is a string of at least 3 characters?
  • email is a real email?
  • age is a number, between 0 and 110?

The best way to handle validating any kind of input coming from outside in Express is by using the express-validator package:

npm install express-validator

You require the check object from the package:

const { check } = require('express-validator/check')

We pass an array of check() calls as the second argument of the post() call. Every check() call accepts the parameter name as argument:

app.post('/form', [
  check('name').isLength({ min: 3 }),
  check('email').isEmail(),
  check('age').isNumeric()
], (req, res) => {
  const name  = req.body.name
  const email = req.body.email
  const age   = req.body.age
})

Notice I used

  • isLength()
  • isEmail()
  • isNumeric()

There are many more of these methods, all coming from validator.js, including:

  • contains(), check if value contains the specified value
  • equals(), check if value equals the specified value
  • isAlpha()
  • isAlphanumeric()
  • isAscii()
  • isBase64()
  • isBoolean()
  • isCurrency()
  • isDecimal()
  • isEmpty()
  • isFQDN(), is a fully qualified domain name?
  • isFloat()
  • isHash()
  • isHexColor()
  • isIP()
  • isIn(), check if the value is in an array of allowed values
  • isInt()
  • isJSON()
  • isLatLong()
  • isLength()
  • isLowercase()
  • isMobilePhone()
  • isNumeric()
  • isPostalCode()
  • isURL()
  • isUppercase()
  • isWhitelisted(), checks the input against a whitelist of allowed characters

You can validate the input against a regular expression using matches().

Dates can be checked using

  • isAfter(), check if the entered date is after the one you pass
  • isBefore(), check if the entered date is before the one you pass
  • isISO8601()
  • isRFC3339()

For exact details on how to use those validators, refer to https://github.com/chriso/validator.js#validators.

All those checks can be combined by piping them:

check('name')
  .isAlpha()
  .isLength({ min: 10 })

If there is any error, the server automatically sends a response to communicate the error. For example if the email is not valid, this is what will be returned:

{
  "errors": [{
    "location": "body",
    "msg": "Invalid value",
    "param": "email"
  }]
}

This default error can be overridden for each check you perform, using withMessage():

check('name')
  .isAlpha()
  .withMessage('Must be only alphabetical chars')
  .isLength({ min: 10 })
  .withMessage('Must be at least 10 chars long')

What if you want to write your own special, custom validator? You can use the custom validator.

In the callback function you can reject the validation either by throwing an exception, or by returning a rejected promise:

app.post('/form', [
  check('name').isLength({ min: 3 }),
  check('email').custom(email => {
    if (alreadyHaveEmail(email)) {
      throw new Error('Email already registered')
    }
  }),
  check('age').isNumeric()
], (req, res) => {
  const name  = req.body.name
  const email = req.body.email
  const age   = req.body.age
})

The custom validator:

check('email').custom(email => {
  if (alreadyHaveEmail(email)) {
    throw new Error('Email already registered')
  }
})

can be rewritten as

check('email').custom(email => {
  if (alreadyHaveEmail(email)) {
    return Promise.reject('Email already registered')
  }
})