Guards the value to be a type of given type.

const guardType = <T extends Type, Payload extends object = object>(
  value: T,
  type: Types<T>,
  callback?: ResultCallback<T, Payload>,
  payload?: Payload
): value is T => isType(value, type, callback, payload);

Generic type variables


A generic type variable T constrained by generic type Type indicates captured type of the supplied 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 guardType() function from which it captures its value.


value: T

The value of a generic type variable T constrained by generic type Type, by default of the type captured from itself, to guard.

type: Types<T>

The value of string or Constructor<T> type of the Types indicates against which type a given value is checked.

callback?: ResultCallback<T, 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 isT

The return type is a boolean as the result of its statement indicating the value is a generic type variable T by default of the type captured from the supplied value.


The return value is a boolean indicating whether the value is a type from a given type of the Types.

Example usage

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

// Person interface.
interface PersonData { firstName: string; }

// Class.
class Person {}

// Object.
const someone: Person = new Person();

const object: PersonData = { firstName: 'firstName' };
guardType(object, 'object'); // true; return type `value is PersonData`

let letObject: PersonData = { firstName: 'firstName' };
guardType(letObject, 'object'); // true; return type `value is PersonData`

function myPerson(person: Person): Person { return person; }
guardType(myPerson, 'function'); // true; return type `value is (person: Person) => Person`

// string.
const firstName = 'firstName';
guardType(firstName, 'string'); // true; return type `value is "firstName"`
let letFirstName = 'firstName';
guardType(letFirstName, 'string'); // true; return type `value is string`

// number.
const age = 5;
guardType(age, 'number'); // true; return type `value is 5`
let letAge = 5;
guardType(letAge, 'number'); // true; return type `value is number`

// null.
const myNull = null;
guardType(myNull, 'null'); // true; return type `value is null`

// bigint
const oldAge = 1n;
guardType(oldAge, 'bigint'); // true; return type `value is 1n`
let letOldAge = 1n;
guardType(letOldAge, 'bigint'); // true; return type `value is bigint`

// Boolean.
const question = true;
guardType(question, 'boolean'); // true; return type `value is true`
let letQuestion = true;
guardType(letQuestion, 'boolean'); // true; return type `value is true`

// Undefined.
const und = undefined;
guardType(und, 'undefined'); // true; return type `value is undefined`
let letUndefined ;
guardType(letUndefined, 'undefined'); // true; return type `value is undefined`

// Symbol.
guardType(Symbol(firstName), 'symbol'); // true; return type `value is symbol`

// Instance.
guardType(someone, Person); // true; return type `value is Person`

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