Checks if any value is not an undefined type and is not equal to undefined.

const isDefined = <Type, Payload extends object = object>(
  value: Type,
  callback: ResultCallback<Type, Payload> = resultCallback,
  payload?: Payload
): value is Defined<Type> =>
  callback(typeof value !== 'undefined' && value !== undefined, value, payload);

Generic type variables


A generic type variable indicates the captured type of the given value except undefined, which changes to never, via the return type and the value type of the supplied callback function.


The Payload generic type variable constrained by object indicates the type of optional parameter payload of the supplied callback function and payload optional parameter of the isDefined() function from which it captures its value.


value: Type

The value of a generic type variable Type, by default of type captured from itself, to check.

callback: ResultCallback<Type, Payload>

A callback function of ResultCallback type with parameters, the value that has been checked, the result of this check, and payload of generic type variable Payload with optional properties from the provided payload, to handle them before the result return. By default, it uses resultCallback() function.

payload?: Payload

An optional object of the generic type variable Payload is assigned to the payload of the given callback function.

Return type

value is Defined<Type>

The return type is a boolean as the result of its statement indicating the value is a generic type Defined that takes a generic type variable Type of value by default equal to the type captured from the supplied value parameter excepts undefined which changes to never.


The return value is a boolean indicating whether the provided value is defined, not undefined.

Example usage

// Example usage.
import { isDefined } from '@angular-package/type';

const UNDEFINED = undefined;
let defined;

isDefined(UNDEFINED); // Returns `false` as `value is never`
isDefined(defined); // Returns `false` as `value is never`
isDefined(27); // Returns `true` as `value is number`
isDefined('age'); // Returns `true` as `value is string`

Parameters callback and payload

// Callback and payload parameters example usage.
import { isDefined } from '@angular-package/type';

isDefined('age', (result, value, payload) => {
  value // 'age'
  if (payload) {
    result // Returns `true`
    payload.notDefined // Returns `false`
  return result;
}, { notDefined: false }); // Returns `true` as `value is string`

Last updated