Typescript. Check if method/property exists in the variable of unknown type

Question

TS v3.x brought new type: unknown. But it's not very clear how to easily use this type instead of any. Example: you're using some 3rd party library which has no types. And you don't have time to write those types yourself. You need to handle some data provided by that library.

Before unknown:

function handle(data: any) {
   if (data && data.arrayProp && typeof data.arrayProp[Symbol.iterator] === 'function') {
       for (let x of data.arrayProp) {...}
   }
}

With unknown:

function handle(data: unknown) {
   // this line gives TS error: `data` is unknown type
   if (data && data.arrayProp && typeof data.arrayProp[Symbol.iterator]=== 'function') {  
...

Most docs in internet are using kind of instanceof ways to check what type data has. But i'm not really interested what type data has. Everything i want to know is if there's arrayProp there. that's it

How to do this with unknown type?

Answer 1

The thing with unknown is that you have to narrow its type before you can use it. You can use a custom type guard for this:

interface ArrayProp {
  arrayProp: []
}
function isArrayProps(value: unknown): value is ArrayProp {
  return !!value && !!(value as ArrayProp).arrayProp;
}

function handle(data: unknown) {
   if (isArrayProps(data) && typeof data.arrayProp[Symbol.iterator] === 'function') {
   }
}

Playground