> For the complete documentation index, see [llms.txt](https://docs.angular-package.dev/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.angular-package.dev/type/is/isstring.md).

# ★ isString()

## `isString()`

Checks if [`any`](https://www.typescriptlang.org/docs/handbook/basic-types.html#any) value is a [`string`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String) type(by using the [`isStringType()`](/type/is/isstringtype.md)) or an instance of [`String`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String) by using the [`isStringObject()`](/type/is/isstringobject.md) function.

{% code title="is-string.func.ts" %}

```typescript
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);
```

{% endcode %}

### Generic type variables

#### <mark style="color:green;">**`Type`**</mark>**`extends`**<mark style="color:green;">**`AnyString`**</mark>**`=`**<mark style="color:green;">**`string`**</mark>

A generic type variable `Type` constrained by [`AnyString`](/type/type/anystring.md) indicates [`string`](https://www.typescriptlang.org/docs/handbook/basic-types.html#string) type of the given [`value`](#value-any) via the [return type](#return-type), by default [`string`](https://www.typescriptlang.org/docs/handbook/basic-types.html#string).

#### <mark style="color:green;">**`Payload`**</mark>**`extends`**<mark style="color:green;">**`object`**</mark>**`=`**<mark style="color:green;">**`object`**</mark>

The `Payload` generic type variable constrained by [`object`](https://www.typescriptlang.org/docs/handbook/basic-types.html#object) indicates the type of optional parameter [`payload`](/type/type/resultcallback.md#payload-payload) of the supplied [`callback`](#callback-resultcallback-less-than-any-payload-greater-than) function and [`payload`](#payload-payload) optional parameter of the [`isString()`](#isstring) function from which it captures its value.

### Parameters

#### `value: any`

The value of [`any`](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#any) type to check.

#### `callback: ResultCallback<any, Payload>`

A callback `function` of [`ResultCallback`](/type/type/resultcallback.md) type with parameters, the [`value`](#value-any) that has been checked, the [`result`](/type/type/resultcallback.md#result-boolean) of this check, and [`payload`](/type/type/resultcallback.md#payload-payload) of generic type variable [`Payload`](#payloadextendsobject-object) with optional properties from the provided [`payload`](#payload-payload), to handle them before the [`result`](/type/type/resultcallback.md#result-boolean) return. By default, it uses [`resultCallback()`](/type/helper/resultcallback.md) function.

#### `payload?: Payload`

An optional [`object`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object) of the generic type variable [`Payload`](#payloadextendsobject-object) is assigned to the [`payload`](/type/type/resultcallback.md#payload-payload) of the given [`callback`](#callback-resultcallback-less-than-any-payload-greater-than) function.

### Return type

#### `value is Type`

The **return type** is a [`boolean`](https://www.typescriptlang.org/docs/handbook/basic-types.html#boolean) as the result of its statement indicating the [`value`](#value-any) is a generic type variable [`Type`](#typeextendsanystring-string) by default equal to the [`string`](https://www.typescriptlang.org/docs/handbook/basic-types.html#string).

### Returns

The **return value** is a [`boolean`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean) indicating whether the provided [`value`](#value-any) is a [`string`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String) type or an instance of [`String`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String).

## Example usage

```typescript
// 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"
```