Path walker

Traveler.ts

export function walkPath<T = any>(path: string[] | string): (source: any) => T | undefined {
  const steps = Array.isArray(path) ? path : path.split('.')
  return (source: any) => steps
    .reduce((node, name) => (node && name in node ? node[name] : undefined), source)
}

export function runPath<T = any>(path: string[] | string): (source: any) => T {
  const steps = Array.isArray(path) ? path : path.split('.')
  return (source: any) => {
    const value = steps.reduce((node, name) =>
      (node && name in node ? node[name] : undefined), source)
    return failLoudly(value)
  }
}

function failLoudly<T = any>(value: T | undefined): T {
  if (typeof value !== 'undefined')
    return value
  else
    throw new Error("Undefined value")
}

// Example:
  
const getTitle = walkPath<string>('user.posts.0.title')
const getSlug = runPath<string>('user.posts.0.slug')

const title = getTitle({})
const slug = getSlug({})
  
slug.toLowerCase() // Allowed, because it won't return undefined

if (title) { // Title could be undefined, so guard it
  title.toUpperCase()
}
else {
  console.error("No title!")
}