★ isString()


Checks if any value is a string type(by using the isStringType()) or an instance of String by using the isStringObject() function.

const isString = <
  Type extends AnyString = string,
  Payload extends object = object
  value: any,
  callback: ResultCallback<any, Payload> = resultCallback,
  payload?: Payload
): value is Type =>
  callback(isStringType(value) || isStringObject(value), value, payload);

Generic type variables


A generic type variable Type constrained by AnyString indicates string type of the given value via the return type, by default string.


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 isString() function from which it captures its value.


value: any

The value of any type to check.

callback: ResultCallback<any, 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 Type

The return type is a boolean as the result of its statement indicating the value is a generic type variable Type by default equal to the string.


The return value is a boolean indicating whether the provided value is a string type or an instance of String.

Example usage

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

isString('age'); // true; The return type `value is string`
isString(new String('age')); // true; The return type `value is string`

// Fake string example.
const fakeString = new Number('asd');

Object.assign(fakeString, {
  get [Symbol.toStringTag](): string {
    return 'string';

isString(fakeString); // false
typeOf(fakeString); // "string"
typeof fakeString; // "object"

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