What does is opeator/keyword do in TS?

[crystal trellis]

function isFunction(value: unknown): value is Function {
  return typeof value === 'function';
}

I also don't understand how the return type is not just boolean looking at the return in the body.

[limpid echo]

It makes the thing passed into value a Function in the place where the function was called

declare const v: V
If (isFunction(v)) {
  // v is now V & Function
}

Although sometimes TS will do this automatically without you needing to write in that return type.

[umbral pier]

Take a look at the playground here @crystal trellis https://tsplay.dev/wg179m

[tiny sigil]

mjuksel#0

Playground Link

Preview:```ts
function ensureStringType(
value: any
): value is string {
return typeof value === "string"
}

let someNullish = "hey" as string | undefined
// ^?

let _length = someNullish.length
// 'someNullish' is possibly 'undefined'.

if (!ensureStringType(someNullish))
...```

You can choose specific lines to embed by selecting them before copying the link.

[hybrid cypress]

!hb predicate

[tiny sigil]

Handbook / Narrowing

Using type predicates

[hybrid cypress]

that function is a predicate ^, aka a type guard

@crystal trellis

[crystal trellis]

Are predicates useful while creating user defined type guard only?

[hybrid cypress]

predicates are typeguards

just different names