Type for non-falsy, aka truthy

Question

In TypeScript, is there a type for truthy?

I have this method: Object.keys(lck.lockholders).length; enqueue(k: any, obj?: any): void think with TS there is a

Answer 1

There is no Truthy type, BUT you can leverage type guard system (type predicate) to help TypeScript to understand what really is truthy and assign it truthy type!

Let's define Falsy type and generic isTruthy function:

type Falsy = false | 0 | '' | null | undefined;

// this is a type predicate - if x is `truthy`, then it's T
const isTruthy = (x: T | Falsy): x is T => !!x;

Now we can use out isTruthy function to find truthy values and TypeScript will correctly assign the "truthy" type to the result.

Examples:

{   // removing boolean "false" type
  const result: string | false = Math.random() > 0.5 ? 'a' : false;
  const truthyResult: string = isTruthy(result) ? result : 'was false';
}
{   // filtering only numbers from array
  const result: (number | undefined)[] = [42, undefined, 8, 16];
  const truthyResult: number[] = result.filter(isTruthy);
}
{   // filtering only Promises from array
  const result: (Promise | null)[] = [Promise.resolve('a'), null, Promise.resolve('b'), Promise.resolve('c')];
  const truthyResult: Promise[] = result.filter(isTruthy);
}