Typescript: method to check if value is: null, undefined, '', [], or {}

Question

I want set the return type of this method to check if the value is: null, undefined, '', [], or {}.

/**
 * Return true if value is a "empty" val, eg: null, undefined, '', [], {}, 0
 */
export const isNullOrUndefinedOrEmpty = (value: any): value is null | undefined | '' | [] | {} | 0 => {
  if (value === null || value === undefined || value === '' || value === 0) {
    return true;
  }
  if (typeof value === 'number' && isNaN(value)) {
    return true;
  }
  if (typeof value === 'object') {
    if (Array.isArray(value)) {
      return value.length <= 0;
    } else {
      return Object.keys(value).length === 0 && value.constructor === Object;
    }
  }
  return false;
};

Usage

const testEmpty = {};
if( !isNullOrUndefinedOrEmpty(testEmpty) ){
    alert(testEmpty.a)
}

const testNotEmpty = {a: true};
if( !isNullOrUndefinedOrEmpty(testNotEmpty) ){
    alert(testNotEmpty.a)
}

But, ts raise this error on both example?

Property 'a' does not exist on type 'never'.

typeof value === 'object' does not check if object has 0 keys (that is {}). Use Object(o).keys.length === 0 when o is non empty object — O-9
– O-9, Commented Oct 19, 2022 at 12:48
Interestingly, the compilation error disappears in the second example with testNotEmpty when the negation (!) is removed. — samjaf
– samjaf, Commented Oct 19, 2022 at 12:52

Bbrk24 · Accepted Answer · 2022-10-19 12:57:23Z

2

The type {} in TypeScript does not mean "empty object", it means "any value besides null or undefined".

The library type-fest provides an EmptyObject type:

declare const emptyObjectSymbol: unique symbol;
type EmptyObject = {[emptyObjectSymbol]?: never};

source: https://github.com/sindresorhus/type-fest/blob/main/source/empty-object.d.ts

answered Oct 19, 2022 at 12:57

Bbrk24

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2 Comments

k.tten Over a year ago

Alternatively Record<never, never>

Bbrk24 Over a year ago

Quote from the documentation on EmptyObject: " Unfortunately, Record<string, never>, Record<keyof any, never> and Record<never, never> do not work. See {@link github.com/sindresorhus/type-fest/issues/395 #395}."

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