# isDefined()

## `isDefined()`

Checks if [`any`](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#any) value is not an [`undefined`](https://developer.mozilla.org/en-US/docs/Glossary/undefined) type and is **not** equal to [`undefined`](https://developer.mozilla.org/en-US/docs/Glossary/undefined).

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

```typescript
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);
```

{% endcode %}

### Generic type variables

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

A generic type variable indicates the captured type of the given [`value`](#value-type) except [`undefined`](https://www.typescriptlang.org/docs/handbook/basic-types.html#null-and-undefined), which changes to [`never`](https://www.typescriptlang.org/docs/handbook/basic-types.html#never), via the [return type](#return-type) and the [`value`](https://type.angular-package.dev/type/resultcallback#value-value) type of the supplied [`callback`](#callback-resultcallback-less-than-type-payload-greater-than) function.&#x20;

#### <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`](https://type.angular-package.dev/type/resultcallback#payload-payload) of the supplied [`callback`](#callback-resultcallback-less-than-type-payload-greater-than) function and [`payload`](#payload-payload) optional parameter of the [`isDefined()`](#isdefined) function from which it captures its value.

### Parameters

#### `value: Type`

The value of a generic type variable [`Type`](#type), by default of type captured from itself, to check.

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

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

### Return type

#### `value is Defined<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-type) is a generic type [`Defined`](https://type.angular-package.dev/type/defined) that takes a generic type variable [`Type`](#type) of value by default equal to the type captured from the supplied [`value`](#value-type) parameter excepts [`undefined`](https://www.typescriptlang.org/docs/handbook/basic-types.html#null-and-undefined) which changes to [`never`](https://www.typescriptlang.org/docs/handbook/basic-types.html#never).

### 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-type) is defined, not [`undefined`](https://developer.mozilla.org/en-US/docs/Glossary/undefined).

## Example usage

```typescript
// 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`

```typescript
// 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`
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://type.angular-package.dev/is/isdefined.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.