Guards the value to be defined, not undefined.

const guardDefined = <Type, Payload extends object = object>(
  value: Defined<Type>,
  callback?: ResultCallback<Defined<Type>, Payload>,
  payload?: Payload
): value is Defined<Type> => isDefined(value, callback, payload);

Generic type variables


A generic type variable Type indicates captured type of the given value via the return type and the value parameter of the provided callback function ResultCallback type.


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 guardDefined() function from which it captures its value.


value: Defined<Type>

The value of generic type Defined<Type>, never undefined type captured from itself to guard against undefined.

callback?: ResultCallback<Defined<Type>, Payload>

The optional 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 value is defined.

Example usage

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

let letFirstName = 'my name';
guardDefined(letFirstName); // true; return type `value is string`

const firstName = 'my const name';
guardDefined(firstName); // true; return type `value is string`

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