themisterholliday/vscode-ts-type-guard-generator

TS Type Guard Generator

Usage

• Select the entirety of an interface defintiion.
• Open your command palette
• Run the command Type Guard Generator
• If everything was okay, the command should generate code at the bottom of your file.
• Import the proper functions

Running

• Run npm install in terminal to install dependencies
• Run the Run Extension target in the Debug View. This will:
  • Start a task npm: watch to compile the code
  • Run the extension in a new VS Code window

Example

1. Start with an interface like this

interface Person {
  stringName: string
  numberName: number
  booleanName: boolean
  stringArray: string[]
  numberArray: number[]
  booleanArray: boolean[]
}

2. Select the entire interface
3. Run the Type Guard Generator command
4. Get a nice generated function like this:

function isPerson(value: unknown): value is Person {

  if (!isPlainObject(value)) return false

  if (!value.hasOwn('stringName')) return false
  if (!value.hasOwn('numberName')) return false
  if (!value.hasOwn('booleanName')) return false
  if (!value.hasOwn('stringArray')) return false
  if (!value.hasOwn('numberArray')) return false
  if (!value.hasOwn('booleanArray')) return false

  const { stringName, numberName, booleanName, stringArray, numberArray, booleanArray } = value

  if (!isString(stringName)) return false
  if (!isNumber(numberName)) return false
  if (!isBoolean(booleanName)) return false
  if (!isStringArray(stringArray)) return false
  if (!isNumberArray(numberArray)) return false
  if (!isBooleanArray(booleanArray)) return false

  return true
}

5. Use your great new function when type guarding:

cosnt someObject: unknown = {}
if (isPerson(someObject)) {
  // someObject can be consider a Person
}

Required supporting code

To use the generated code you'll need to have all the code below in your project.

type-guard.util.ts

interface PlainObject {
  // Object.hasOwn() is intended as a replacement for Object.hasOwnProperty(). See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn
  hasOwn<K extends string>(key: K): this is Record<K, unknown>
}

export function isPlainObject(value: unknown): value is PlainObject {
  return !!value && typeof value === 'object' && !Array.isArray(value)
}

export function isBoolean(value: unknown): value is boolean {
  return typeof value === "boolean"
}

export function isString(value: unknown): value is string {
  return typeof value === "string"
}

export function isNumber(value: unknown): value is number {
  return typeof value === "number"
}

export function isBooleanArray(value: unknown): value is boolean[] {
  if (!Array.isArray(value)) return false
  for (const item of value) {
    if (!isBoolean(item)) {
      return false
    }
  }
  return true
}

export function isStringArray(value: unknown): value is string[] {
  if (!Array.isArray(value)) return false
  for (const item of value) {
    if (!isString(item)) {
      return false
    }
  }
  return true
}

export function isNumberArray(value: unknown): value is number[] {
  if (!Array.isArray(value)) return false
  for (const item of value) {
    if (!isNumber(item)) {
      return false
    }
  }
  return true
}

import statements

import {
  isPlainObject,
  isBoolean,
  isString,
  isNumber,
  isBooleanArray,
  isStringArray,
  isNumberArray,
} from "./type-guard.util"