ryannhg/safe-json

Safely handle unknown JSON in Typescript

installation

npm install @ryannhg/safe-json

the problem

When our applications receive data from randos on the internet, we don't know what to expect! With Typescript, the easiest way to handle this uncertainty is by using the any keyword. For example, Express does this for req.body.

This leads to one minor issue... it breaks our entire type system!

const increment = (a: number) => a + 1

const data : any = { counter: '2' }
const value = increment(data.counter)

console.log(value) // "21"

That any type broke the safety of our increment function!

What's even worse? TypeScript thinks value is a number now! Ah! It's like we're just using JS again!!

an ideal solution

What should we do instead?

The unknown JSON from before should really be treated as an unknown. The unknown type reminds us to check our JSON before passing it around, so it won't break everything like a sneaky snek! 🐍

Here's the same code from before, but using unknown:

const increment = (a: number) => a + 1

const data : unknown = { counter: '2' }
const value = increment(data.counter) // Type error!

We need to convert the unknown to a { counter : number } type.

Unfortunately, working with unknown values is a pain. Proving that data is an object is easy, but Typescript yells when accessing properties like counter. Most handwritten solutions involve using any or as keywords, which is the whole situation we are trying to avoid!

the solution

This is where a smaller library can save us a lot of headache.

import { Expect, Validator } from '@ryannhg/safe-json'

const increment = (a: number) => a + 1

const data : unknown = { counter: '2' }

// Step 1. Define the type we expect
type OurData = {
  counter: number
}

// Step 2. Define a validator
const ourValidator : Validator<OurData> =
  Expect.object({
    counter: Expect.number
  })

// Step 3. Validate the unknown data
if (ourValidator.worksWith(data)) {
  // ✅ `data` is now the "OurData" type
  const value = increment(data.counter)
}

API

Ready to try it out? Theres's not much to learn!

Creating Validators

• Expect.boolean
• Expect.number
• Expect.string
• Expect.null
• Expect.object
• Expect.array
• Expect.optional

Validating JSON

• validator.worksWith
• validator.run

Expect.boolean

Safely handle boolean values.

Expect.boolean : Validator<boolean>

Expect.boolean.worksWith(true)       // ✅
Expect.boolean.worksWith(false)      // ✅
Expect.boolean.worksWith(undefined)  // 🚫
Expect.boolean.worksWith('true')     // 🚫
Expect.boolean.worksWith(null)       // 🚫
Expect.boolean.worksWith(0)          // 🚫

Expect.number

Safely handle number values.

Expect.number : Validator<number>

Expect.number.worksWith(123)        // ✅
Expect.number.worksWith(2.5)        // ✅
Expect.number.worksWith(-12)        // ✅
Expect.number.worksWith(0)          // ✅
Expect.number.worksWith('12')       // 🚫
Expect.number.worksWith(null)       // 🚫

Expect.string

Safely handle string values.

Expect.string : Validator<string>

Expect.string.worksWith('123')        // ✅
Expect.string.worksWith('true')       // ✅
Expect.string.worksWith(123)          // 🚫
Expect.string.worksWith(true)         // 🚫
Expect.string.worksWith(undefined)    // 🚫
Expect.string.worksWith(null)         // 🚫

Expect.null

Safely handle null values.

Expect.null : Validator<null>

Expect.null.worksWith(null)       // ✅
Expect.null.worksWith(undefined)  // 🚫
Expect.null.worksWith('null')     // 🚫
Expect.null.worksWith(false)      // 🚫
Expect.null.worksWith(0)          // 🚫

Expect.object

Safely handle object values. Provide an object mapping field name to any other Validator. You can even reuse validators you defined before!

Expect.object : <T>(fields: Fields<T>) => Validator<T>

type Person = { name: string, age: number }

const person: Validator<Person> =
  Expect.object({
    name: Expect.string,
    age: Expect.number
  })

person.worksWith({ name: 'ryan', age: 26 })   // ✅
person.worksWith({ name: 'ryan', age: "26" }) // 🚫
person.worksWith({ nam: 'ryan',  age: 26 })   // 🚫
person.worksWith({ name: 'ryan' })            // 🚫
person.worksWith({ age: 26 })                 // 🚫
person.worksWith(null)                        // 🚫

Expect.array

Safely handle array values of the same type!

Expect.array : <T>(validator: Validator<T>) => Validator<T[]>

Expect.array(Expect.number).worksWith([])             // ✅
Expect.array(Expect.number).worksWith([ 1, 2, 3 ])    // ✅
Expect.array(Expect.number).worksWith([ 1, null, 3 ]) // 🚫
Expect.array(Expect.number).worksWith([ 1, 2, '3' ])  // 🚫
Expect.array(Expect.number).worksWith(null)           // 🚫

Expect.optional

Allows a value to be optional. Always succeeds, but is undefined if the value couldn't be parsed from the JSON.

Expect.optional : <T>(validator: Validator<T>) => Validator<T | undefined>

const maybeNumber : Validator<number | undefined> =
  Expect.optional(Expect.number)

maybeNumber.worksWith(123)        // ✅ (123)
maybeNumber.worksWith(456)        // ✅ (456)
maybeNumber.worksWith(null)       // ✅ (undefined)
maybeNumber.worksWith(undefined)  // ✅ (undefined)
maybeNumber.worksWith(true)       // ✅ (undefined)

validator.worksWith

Allows you to test your unknown data against a Validator<T>. If the worksWith function returns true, the data is guaranteed to be the correct type.

worksWith: (data: unknown) => data is value

type Person = { name : string }

const person : Validator<Person> =
  Expect.object({
    name: Expect.string
  })

✅ Pass Example

const data = { name: "Ryan" }

if (person.worksWith(data)) {
  console.log(data.name)
} else {
  console.error('Not a person!')
}

This code prints "Ryan", because the data passed validation.

🚫 Fail Example

const data = { name: null }

if (person.worksWith(data)) {
  console.log(data.name)
} else {
  console.error('Not a person!')
}

This code prints "Not a person!", because the data failed validation.

validator.run

The run function is another way to handle the branching logic, or provide a fallback if you'd like.

In the event of a failure, it also provides a reason that the JSON failed validation!

run: <T, U>(data: unknown, handlers: {
    onPass: (value: value) => T,
    onFail: (reason: Problem) => U
  }) => T | U

type Person = { name : string }

const person : Validator<Person> =
  Expect.object({
    name: Expect.string
  })

✅ Pass Example

person.run({ name: "Ryan" }, {
  onPass: person => console.log(person.name),
  onFail: reason => console.error(reason)
})

This code prints "Ryan", because the data passed validation.

🚫 Fail Example

person.run({ name: null }, {
  onPass: person => console.log(person.name),
  onFail: reason => console.error(reason)
})

This code prints

'Problem with field "name": Expecting a string, but got null.'

because the data failed validation.

inspiration

Like all good things in my life, I stole it from Elm. There's a package called elm/json that converts raw JSON from the outside world into reliable values you can trust in your application.

Check out that package here:

https://package.elm-lang.org/packages/elm/json/latest/