Config Parser for Node.js

configParser.ts

import Yaml from 'js-yaml'
import 'reflect-metadata'

export interface SettingOptions {
  nullable?: boolean
}

export const Setting = ({ nullable = false }: SettingOptions = {}): PropertyDecorator => (target, propertyKey) => {
  let properties: string[] = Reflect.getMetadata('config:settings', target)
  if (!properties) {
    properties = []
    Reflect.defineMetadata('config:settings', properties, target)
  }
  properties.push(propertyKey as string)

  Reflect.defineMetadata('config:nullable', nullable, target, propertyKey)
}

export type ObjectType<T> = { new(): T };

const toSnakeCase = (property: string): string => {
  return property.replace(/[A-Z]/g, (s) => `_${s.toLowerCase()}`)
}

const ensureString = (value: unknown): string => {
  if (typeof value === 'string') {
    return value
  } else if (typeof value === 'number') {
    return String(value)
  }
  throw new Error('Cannot parse value as string: ' + value)
}

const ensureNumber = (value: unknown): number => {
  if (typeof value === 'number') {
    return value
  }
  throw new Error('Cannot parse value as number: ' + value)
}

const ensureBoolean = (value: unknown): boolean => {
  if (typeof value === 'boolean') {
    return value
  }
  throw new Error('Cannot parse value as boolean: ' + value)
}

const ensureDate = (value: unknown): Date => {
  if (value instanceof Date) {
    return value
  }
  throw new Error('Cannot parse value as date: ' + value)
}

export const parseConfig = <T extends object>(yaml: string, target: ObjectType<T>): T => {
  const doc = Yaml.safeLoad(yaml)
  console.log(doc)

  return parseDoc(doc, target)
}

export const parseDoc = <T extends object>(doc: any, target: ObjectType<T>): T => {
  const object = new target()
  const properties: string[] = Reflect.getMetadata('config:settings', object) || []

  for (const property of properties) {
    const subDoc = doc[toSnakeCase(property)]
    const type = Reflect.getMetadata('design:type', object, property)
    const nullable: boolean = Reflect.getMetadata('config:nullable', object, property)

    const prop = (value: any) => Reflect.defineProperty(object, property, { value, enumerable: true })

    if ((typeof subDoc === 'undefined' || subDoc === null) && nullable) {
      prop(undefined)
    } else if (type === String) {
      prop(ensureString(subDoc))
    } else if (type === Number) {
      prop(ensureNumber(subDoc))
    } else if (type === Boolean) {
      prop(ensureBoolean(subDoc))
    } else if (type === Date) {
      prop(ensureDate(subDoc))
    } else {
      console.log(property, type)
      prop(parseDoc(subDoc, type))
    }
  }

  return object
}

example.ts

import { parseConfig, Setting } from './configParser'

const yaml = `
demo:
  test: Hello
  number: 42
  bool: ~
  hello_world: "demo"
date: 2019-07-18
`

class Demo {
  @Setting()
  test!: string

  @Setting()
  number!: number

  @Setting({ nullable: true })
  bool?: boolean

  @Setting()
  helloWorld!: string
}

class Config {
  @Setting()
  demo!: Demo

  @Setting()
  date!: Date
}

const config = parseConfig(yaml, Config)
console.dir(config)

if (config.demo.bool !== undefined) {
  console.dir(config.demo.bool.toString())
}

index.ts

import Yaml from 'js-yaml'
import 'reflect-metadata'

export interface SettingOptions {
  nullable?: boolean
}

export const Setting = ({ nullable = false }: SettingOptions = {}): PropertyDecorator => (target, propertyKey) => {
  let properties: string[] = Reflect.getMetadata('config:settings', target)
  if (!properties) {
    properties = []
    Reflect.defineMetadata('config:settings', properties, target)
  }
  properties.push(propertyKey as string)

  Reflect.defineMetadata('config:nullable', nullable, target, propertyKey)
}

export type ObjectType<T> = { new(): T };

const toSnakeCase = (property: string): string => {
  return property.replace(/[A-Z]/g, (s) => `_${s.toLowerCase()}`)
}

const ensureString = (value: unknown): string => {
  if (typeof value === 'string') {
    return value
  } else if (typeof value === 'number') {
    return String(value)
  }
  throw new Error('Cannot parse value as string: ' + value)
}

const ensureNumber = (value: unknown): number => {
  if (typeof value === 'number') {
    return value
  }
  throw new Error('Cannot parse value as number: ' + value)
}

const ensureBoolean = (value: unknown): boolean => {
  if (typeof value === 'boolean') {
    return value
  }
  throw new Error('Cannot parse value as boolean: ' + value)
}

const ensureDate = (value: unknown): Date => {
  if (value instanceof Date) {
    return value
  }
  throw new Error('Cannot parse value as date: ' + value)
}

export const parseConfig = <T extends object>(yaml: string, target: ObjectType<T>): T => {
  const doc = Yaml.safeLoad(yaml)
  console.log(doc)

  return parseDoc(doc, target)
}

export const parseDoc = <T extends object>(doc: any, target: ObjectType<T>): T => {
  const object = new target()
  const properties: string[] = Reflect.getMetadata('config:settings', object) || []

  for (const property of properties) {
    const subDoc = doc[toSnakeCase(property)]
    const type = Reflect.getMetadata('design:type', object, property)
    const nullable: boolean = Reflect.getMetadata('config:nullable', object, property)

    const prop = (value: any) => Reflect.defineProperty(object, property, { value, enumerable: true })

    if ((typeof subDoc === 'undefined' || subDoc === null) && nullable) {
      prop(undefined)
    } else if (type === String) {
      prop(ensureString(subDoc))
    } else if (type === Number) {
      prop(ensureNumber(subDoc))
    } else if (type === Boolean) {
      prop(ensureBoolean(subDoc))
    } else if (type === Date) {
      prop(ensureDate(subDoc))
    } else {
      console.log(property, type)
      prop(parseDoc(subDoc, type))
    }
  }

  return object
}