xref: /napi_generator/examples/napitutorials/doc/apiguide/object.md

# Object

Class `Napi::Object` inherits from class [`Napi::TypeTaggable`][].

The `Napi::Object` class corresponds to a JavaScript object. It is extended by the following node-addon-api classes that you may use when working with more specific types:

- [`Napi::Array`](array.md)
- [`Napi::ArrayBuffer`](array_buffer.md)
- [`Napi::Buffer<T>`](buffer.md)
- [`Napi::Function`](function.md)
- [`Napi::TypedArray`](typed_array.md).

This class provides a number of convenience methods, most of which are used to set or get properties on a JavaScript object. For example, Set() and Get().

## Example
```cpp
#include <napi.h>

using namespace Napi;

Void Init(Env env) {

  // Create a new instance
  Object obj = Object::New(env);

  // Assign values to properties
  obj.Set("hello", "world");
  obj.Set(uint32_t(42), "The Answer to Life, the Universe, and Everything");
  obj.Set("Douglas Adams", true);

  // Get properties
  Value val1 = obj.Get("hello");
  Value val2 = obj.Get(uint32_t(42));
  Value val3 = obj.Get("Douglas Adams");

  // Test if objects have properties.
  bool obj1 = obj.Has("hello"); // true
  bool obj2 = obj.Has("world"); // false

}
```

## Methods

### Empty Constructor

```cpp
Napi::Object::Object();
```
Creates a new empty Object instance.

### Constructor

```cpp
Napi::Object::Object(napi_env env, napi_value value);
```
- `[in] env`: The `napi_env` environment in which to construct the Value object.

- `[in] value`: The `napi_value` which is a handle for a JavaScript object.

Creates a non-empty `Napi::Object` instance.

### New()

```cpp
Napi::Object Napi::Object::New(napi_env env);
```
- `[in] env`: The `napi_env` environment in which to construct the `Napi::Value` object.

Creates a new `Napi::Object` value.

### Set()

```cpp
bool Napi::Object::Set (____ key, ____ value) const;
```
- `[in] key`: The name for the property being assigned.
- `[in] value`: The value being assigned to the property.

Add a property with the specified key with the specified value to the object.

The key can be any of the following types:
- `napi_value`
- [`Napi::Value`](value.md)
- `const char*`
- `const std::string&`
- `uint32_t`

The `value` can be of any type that is accepted by [`Napi::Value::From`][].

### Delete()

```cpp
bool Napi::Object::Delete(____ key) const;
```
- `[in] key`: The name of the property to delete.

Deletes the property associated with the given key. Returns `true` if the property was deleted.

The `key` can be any of the following types:
- `napi_value`
- [`Napi::Value`](value.md)
- `const char *`
- `const std::string &`
- `uint32_t`

### Get()

```cpp
Napi::Value Napi::Object::Get(____ key);
```
- `[in] key`: The name of the property to return the value for.

Returns the [`Napi::Value`](value.md) associated with the key property. Returns the value *undefined* if the key does not exist.

The `key` can be any of the following types:
- `napi_value`
- [`Napi::Value`](value.md)
- `const char *`
- `const std::string &`
- `uint32_t`

### Has()

```cpp
bool Napi::Object::Has (____ key) const;
```
- `[in] key`: The name of the property to check.

Returns a `bool` that is *true* if the object has a property named `key` and *false* otherwise.

### InstanceOf()

```cpp
bool Napi::Object::InstanceOf (const Function& constructor) const
```
- `[in] constructor`: The constructor [`Napi::Function`](function.md) of the value that is being compared with the object.

Returns a `bool` that is true if the `Napi::Object` is an instance created by the `constructor` and false otherwise.

Note: This is equivalent to the JavaScript instanceof operator.

### AddFinalizer()
```cpp
template <typename Finalizer, typename T>
inline void AddFinalizer(Finalizer finalizeCallback, T* data) const;
```

- `[in] finalizeCallback`: The function to call when the object is garbage-collected.
- `[in] data`: The data to associate with the object.

Associates `data` with the object, calling `finalizeCallback` when the object is garbage-collected. `finalizeCallback`
has the signature
```cpp
void finalizeCallback(Napi::Env env, T* data);
```
where `data` is the pointer that was passed into the call to `AddFinalizer()`.

### AddFinalizer()
```cpp
template <typename Finalizer, typename T, typename Hint>
inline void AddFinalizer(Finalizer finalizeCallback,
                         T* data,
                         Hint* finalizeHint) const;
```

- `[in] data`: The data to associate with the object.
- `[in] finalizeCallback`: The function to call when the object is garbage-collected.

Associates `data` with the object, calling `finalizeCallback` when the object is garbage-collected. An additional hint
may be given. It will also be passed to `finalizeCallback`, which has the signature
```cpp
void finalizeCallback(Napi::Env env, T* data, Hint* hint);
```
where `data` and `hint` are the pointers that were passed into the call to `AddFinalizer()`.

### GetPropertyNames()
```cpp
Napi::Array Napi::Object::GetPropertyNames() const;
```

Returns the names of the enumerable properties of the object as a [`Napi::Array`](array.md) of strings.
The properties whose key is a `Symbol` will not be included.

### HasOwnProperty()
```cpp
bool Napi::Object::HasOwnProperty(____ key) const;
```
- `[in] key` The name of the property to check.

Returns a `bool` that is *true* if the object has an own property named `key` and *false* otherwise.

The key can be any of the following types:
- `napi_value`
- [`Napi::Value`](value.md)
- `const char*`
- `const std::string&`
- `uint32_t`

### DefineProperty()

```cpp
bool Napi::Object::DefineProperty (const Napi::PropertyDescriptor& property) const;
```
- `[in] property`: A [`Napi::PropertyDescriptor`](property_descriptor.md).

Define a property on the object.

### DefineProperties()

```cpp
bool Napi::Object::DefineProperties (____ properties) const;
```
- `[in] properties`: A list of [`Napi::PropertyDescriptor`](property_descriptor.md). Can be one of the following types:
  - const std::initializer_list<Napi::PropertyDescriptor>&
  - const std::vector<Napi::PropertyDescriptor>&

Defines properties on the object.

### Freeze()

```cpp
void Napi::Object::Freeze() const;
```

The `Napi::Object::Freeze()` method freezes an object. A frozen object can no
longer changed. Freezing an object prevents new properties from being added to
it, existing properties from being removed, prevents changing the
enumerability, configurability, or writability of existing properties and
prevents the value of existing properties from being changed. In addition,
freezing an object also prevents its prototype from being changed.

### Seal()

```cpp
void Napi::Object::Seal() const;
```

The `Napi::Object::Seal()` method seals an object, preventing new properties
from being added to it and marking all existing properties as non-configurable.
Values of present properties can still be changed as long as they are
writable.

### operator\[\]()

```cpp
Napi::PropertyLValue<std::string> Napi::Object::operator[] (const char* utf8name) const;
```
- `[in] utf8name`: UTF-8 encoded null-terminated property name.

Returns a [`Napi::Object::PropertyLValue`](propertylvalue.md) as the named
property or sets the named property.

```cpp
Napi::PropertyLValue<std::string> Napi::Object::operator[] (const std::string& utf8name) const;
```
- `[in] utf8name`: UTF-8 encoded property name.

Returns a [`Napi::Object::PropertyLValue`](propertylvalue.md) as the named
property or sets the named property.

```cpp
Napi::PropertyLValue<uint32_t> Napi::Object::operator[] (uint32_t index) const;
```
- `[in] index`: Element index.

Returns a [`Napi::Object::PropertyLValue`](propertylvalue.md) or sets an
indexed property or array element.

### begin()

```cpp
Napi::Object::iterator Napi::Object::begin() const;
```

Returns a constant iterator to the beginning of the object.

```cpp
Napi::Object::iterator Napi::Object::begin();
```

Returns a non constant iterator to the beginning of the object.

### end()

```cpp
Napi::Object::iterator Napi::Object::end() const;
```

Returns a constant iterator to the end of the object.

```cpp
Napi::Object::iterator Napi::Object::end();
```

Returns a non constant iterator to the end of the object.

## Iterator

Iterators expose an `std::pair<...>`, where the `first` property is a
[`Napi::Value`](value.md) that holds the currently iterated key and the
`second` property is a [`Napi::Object::PropertyLValue`](propertylvalue.md) that
holds the currently iterated value. Iterators are only available if C++
exceptions are enabled (by defining `NAPI_CPP_EXCEPTIONS` during the build).

### Constant Iterator

In constant iterators, the iterated values are immutable.

#### operator++()

```cpp
inline Napi::Object::const_iterator& Napi::Object::const_iterator::operator++();
```

Moves the iterator one step forward.

#### operator==

```cpp
inline bool Napi::Object::const_iterator::operator==(const Napi::Object::const_iterator& other) const;
```
- `[in] other`: Another iterator to compare the current iterator to.

Returns whether both iterators are at the same index.

#### operator!=

```cpp
inline bool Napi::Object::const_iterator::operator!=(const Napi::Object::const_iterator& other) const;
```
- `[in] other`: Another iterator to compare the current iterator to.

Returns whether both iterators are at different indices.

#### operator*()

```cpp
inline const std::pair<Napi::Value, Napi::Object::PropertyLValue<Napi::Value>> Napi::Object::const_iterator::operator*() const;
```

Returns the currently iterated key and value.

#### Example
```cpp
Value Sum(const CallbackInfo& info) {
  Object object = info[0].As<Object>();
  int64_t sum = 0;

  for (const auto& e : object) {
    sum += static_cast<Value>(e.second).As<Number>().Int64Value();
  }

  return Number::New(info.Env(), sum);
}
```

### Non Constant Iterator

In non constant iterators, the iterated values are mutable.

#### operator++()

```cpp
inline Napi::Object::iterator& Napi::Object::iterator::operator++();
```

Moves the iterator one step forward.

#### operator==

```cpp
inline bool Napi::Object::iterator::operator==(const Napi::Object::iterator& other) const;
```
- `[in] other`: Another iterator to compare the current iterator to.

Returns whether both iterators are at the same index.

#### operator!=

```cpp
inline bool Napi::Object::iterator::operator!=(const Napi::Object::iterator& other) const;
```
- `[in] other`: Another iterator to compare the current iterator to.

Returns whether both iterators are at different indices.

#### operator*()

```cpp
inline std::pair<Napi::Value, Napi::Object::PropertyLValue<Napi::Value>> Napi::Object::iterator::operator*();
```

Returns the currently iterated key and value.

#### Example
```cpp
void Increment(const CallbackInfo& info) {
  Env env = info.Env();
  Object object = info[0].As<Object>();

  for (auto e : object) {
    int64_t value = static_cast<Value>(e.second).As<Number>().Int64Value();
    ++value;
    e.second = Napi::Number::New(env, value);
  }
}
```

[`Napi::TypeTaggable`]: ./type_taggable.md
[`Napi::Value::From`]: ./value.md#from