Telvis

Typescript/Javascript safe navigation

About

This library adds a path method to the Object prototype which allows you to safely get a nested property of an object. This is comparable to the elvis operator ?. (Swift, C#) or monadic options like Option.flatMap (Scala) or Maybe.#>> (Haskell)

a.path("b").path("c").path("d") will return a.b.c.d if a, b and c and not null or undefined, and otherwise will return undefined. This can also be shortened to a.path("b","c","d")

Note: Requires Typescript 2.1 because it uses the keyof operator

Installation

npm install telvis --save

In your files: (WIP) Sorry, still working out the modules

Examples

interface A { b ?: B }
interface B { c ?: C }
interface C { hello ?: string }

let a1: A = { b : { hello: "world" }}
let a2: A = { b : {} }

// world will be of type string | undefined and at runtime will contain "world"
let world = a1.path("b", "hello")
// noworld will be of type string | undefined and at runtime will contain undefined
let noworld = a2.path("b", "hello")
// This will not compile, as "nonexistent" is not a key in A
// let invalid = a1.path("nonexistent")