Type Alias FalsyValue

FalsyValue: 0 | "" | 0 | 0n | false | null | undefined

Represents the set of JavaScript values that are considered "falsy" in boolean contexts.

In JavaScript, these values are coerced to false when used in boolean contexts such as if statements, logical operators (&&, ||), or the Boolean() constructor.

The falsy values include:

false - the boolean false literal
0 - the number zero
-0 - negative zero (distinct from 0 in some contexts)
0n - the bigint zero
'' - empty string
null - the null value
undefined - the undefined value

Note: NaN is also falsy at runtime but cannot be represented as a literal type since NaN !== NaN and TypeScript cannot create a literal type for it.

Example

const checkFalsy = (value: unknown): value is FalsyValue => {
  return !value;
};

checkFalsy(false);    // true
checkFalsy(0);        // true
checkFalsy('');       // true
checkFalsy(null);     // true
checkFalsy(undefined); // true
checkFalsy('hello');  // false
checkFalsy(1);        // false

// Type guard for filtering out falsy values
const truthyValues = [0, 1, '', 'hello', false, true, null].filter(
  (value): value is Exclude<typeof value, FalsyValue> => Boolean(value)
); // [1, 'hello', true]

// Conditional logic based on falsy values
type IsFalsy<T> = T extends FalsyValue ? true : false;
type Test1 = IsFalsy<0>;     // true
type Test2 = IsFalsy<'hi'>;  // false

Type Alias FalsyValue

Example

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