ts-auto-guard

Generate type guard functions from TypeScript interfaces

A tool for automatically generating TypeScript type guards for interfaces in your code base.

This tool aims to allow developers to verify data from untyped sources to ensure it conforms to TypeScript types. For example when initializing a data store or receiving structured data in an AJAX response.

Install

Yarn

$ yarn add -D ts-auto-guard

npm

$ npm install --save-dev ts-auto-guard

Usage

Specify which types to process (see below) and run the CLI tool in the same folder as your project's tsconfig.json (optionally passing in paths to the files you'd like it to parse).

$ ts-auto-guard ./my-project/Person.ts

See generated files alongside your annotated files:

// my-project/Person.guard.ts

import { Person } from './Person'

export function isPerson(obj: any): obj is Person {
  return (
    typeof obj === 'object' &&
    typeof obj.name === 'string' &&
    (typeof obj.age === 'undefined' || typeof obj.age === 'number') &&
    Array.isArray(obj.children) &&
    obj.children.every(e => isPerson(e))
  )
}

Now use in your project:

// index.ts

import { Person } from './Person'
import { isPerson } from './Person.guard'

// Loading up an (untyped) JSON file
const person = require('./person.json')

if (isPerson(person)) {
  // Can trust the type system here because the object has been verified.
  console.log(`${person.name} has ${person.children.length} child(ren)`)
} else {
  console.error('Invalid person.json')
}

Specifying which types to process

Specify with annotation

Annotate interfaces in your project or pass. ts-auto-guard will generate guards only for interfaces with a @see {name} ts-auto-guard:type-guard JSDoc tag.

// my-project/Person.ts

/** @see {isPerson} ts-auto-guard:type-guard */
export interface Person {
  // !do not forget to export - only exported types are processed
  name: string
  age?: number
  children: Person[]
}

Process all types

Use --export-all parameter to process all exported types:

$ ts-auto-guard --export-all

Debug mode

Use debug mode to help work out why your type guards are failing in development. This will change the output type guards to log the path, expected type and value of failing guards.

$ ts-auto-guard --debug

isPerson({ name: 20, age: 20 })
// stderr: "person.name type mismatch, expected: string, found: 20"

Short circuiting

ts-auto-guard also supports a shortcircuit flag that will cause all guards to always return true.

$ ts-auto-guard --shortcircuit="process.env.NODE_ENV === 'production'"

This will result in the following:

// my-project/Person.guard.ts

import { Person } from './Person'

export function isPerson(obj: any): obj is Person {
  if (process.env.NODE_ENV === 'production') {
    return true
  }
  return (
    typeof obj === 'object' &&
    // ...normal conditions
  )
}

Using the shortcircuit option in combination with uglify-js's dead_code and global_defs options will let you omit the long and complicated checks from your production code.

Return errors

Use --return-errors flag to create an extra function that will not acts as typesafe guards, but output an array of appropriate errors (the same errors as given in the debug mode). Intended use is for example integrity testing non typesafe database outputs, that can then be printed to the screen.

$ ts-auto-guard --return-errors

// Generic usage
isObjErrorOut(obj): errors: Error[]

const returnVar = isPersonErrorOut({ name: "John", age: 20 })
// returnVar.length: 0

const returnVar = isPersonErrorOut({ name: 20, age: 20 })
// returnVar[0].message: "person.name type mismatch, expected: string, found: 20"