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3.1.10 • Public • Published

JSON Type Validation

A TypeScript library to perform type checking and validation on untyped JSON data at runtime.

This library owes thanks to:


npm i @mojotech/json-type-validation

Projects using < typescript@3.0.1 will need a polyfill for the unknown type, such as unknown-ts.


Let's say we're creating a web app for our pet sitting business, and we've picked TypeScript as one of our core technologies. This is a great choice because the extra stability and type safety that TypeScript provides is really going to help us market our business.

We've defined the data we need to track about each client's pet:

interface Pet {
  name: string;
  species: string;
  age?: number;
  isCute?: boolean;

And we've got some data about current client's pets which is stored as JSON:

const croc: Pet = JSON.parse('{"name":"Lyle","species":"Crocodile","isCute":true}')
const moose: Pet = JSON.parse('{"name":"Bullwinkle","age":59}')

But that can't be right -- our data for moose is missing information required for the Pet interface, but TypeScript compiles the code just fine!

Of course this isn't an issue with TypeScript, but with our own type annotations. In TypeScript JSON.parse has a return type of any, which pushes the responsibility of verifying the type of data onto the user. By assuming that all of our data is correctly formed, we've left ourselves open to unexpected errors at runtime.

Unfortunately TypeScript doesn't provide a good built-in way to deal with this issue. Providing run-time type information is one of TypeScript's non-goals, and our web app is too important to risk using a forked version of TypeScript with that added functionality. Type guards work, but are limited in that they circumvent type inference instead of working with it, and can be cumbersome to write.

With json-type-validation we can define decoders that validate untyped json input. Decoders are concise, composable, and typecheck against our defined types and interfaces.

import {Decoder, object, string, optional, number, boolean} from '@mojotech/json-type-validation'

const petDecoder: Decoder<Pet> = object({
  name: string(),
  species: string(),
  age: optional(number()),
  isCute: optional(boolean())

Finally, we can choose from a number of decoding methods to validate json and report success or failure. When some json input fails validation the decoder clearly shows how the data was malformed.

const lyle: Pet = petDecoder.runWithException(croc)

const bullwinkle: Pet = petDecoder.runWithException(moose)
// Throws the exception:
// `Input: {"name":"Bullwinkle","age":59}
//  Failed at input: the key 'species' is required but was not present`




With Nix

There exists some Nix infrastructure that can be used to reproduce a build environment exactly. A helper shell script lives at bin/jtv that you can use to enter environments for multiple uses. You'll need to follow the directions on the Nix website to install and use the Nix package manager.

  • To enter a shell suitable for building the library run ./bin/jtv build-shell. This will leave you in the root of the project and automatically install any project and npm dependencies. You can run further yarn commands here.
  • To build the library for distribution and exit you can run ./bin/jtv distribute.
  • To enter a build shell and run the build process, watching for changes, run ./bin/jtv build-watch.
  • To run an arbitrary command in a build environment use ./bin/jtv run COMMAND. For example, ./bin/jtv run yarn test will run the tests and exit.



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