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# Promise
Class `Napi::Promise` inherits from class [`Napi::Object`][].
The `Napi::Promise` class, along with its `Napi::Promise::Deferred` class, implement the ability to create, resolve, and reject Promise objects.
The basic approach is to create a `Napi::Promise::Deferred` object and return to your caller the value returned by the `Napi::Promise::Deferred::Promise` method. For example:
```cpp
Napi::Value YourFunction(const Napi::CallbackInfo& info) {
// your code goes here...
Napi::Promise::Deferred deferred = Napi::Promise::Deferred::New(info.Env());
// deferred needs to survive this call...
return deferred.Promise();
}
```
Later, when the asynchronous process completes, call either the `Resolve` or `Reject` method on the `Napi::Promise::Deferred` object created earlier:
```cpp
deferred.Resolve(String::New(info.Env(), "OK"));
```
## Promise::Deferred Methods
### Factory Method
```cpp
static Napi::Promise::Deferred Napi::Promise::Deferred::New(napi_env env);
```
* `[in] env`: The `napi_env` environment in which to create the `Napi::Promise::Deferred` object.
### Constructor
```cpp
Napi::Promise::Deferred(napi_env env);
```
* `[in] env`: The `napi_env` environment in which to construct the `Napi::Promise::Deferred` object.
### Env
```cpp
Napi::Env Napi::Promise::Deferred::Env() const;
```
Returns the Env environment this `Napi::Promise::Deferred` object is associated with.
### Promise
```cpp
Napi::Promise Napi::Promise::Deferred::Promise() const;
```
Returns the `Napi::Promise` object held by the `Napi::Promise::Deferred` object.
### Resolve
```cpp
void Napi::Promise::Deferred::Resolve(napi_value value) const;
```
Resolves the `Napi::Promise` object held by the `Napi::Promise::Deferred` object.
* `[in] value`: The Node-API primitive value with which to resolve the `Napi::Promise`.
### Reject
```cpp
void Napi::Promise::Deferred::Reject(napi_value value) const;
```
Rejects the Promise object held by the `Napi::Promise::Deferred` object.
* `[in] value`: The Node-API primitive value with which to reject the `Napi::Promise`.
## Promise Methods
### Then
```cpp
Napi::Promise Napi::Promise::Then(napi_value onFulfilled) const;
Napi::Promise Napi::Promise::Then(const Function& onFulfilled) const;
```
Attaches a fulfillment handler to the promise and returns a new promise.
**Parameters:**
* `[in] onFulfilled`: The fulfillment handler for the promise. May be any of:
- `napi_value` – a JavaScript function to be called when the promise is fulfilled.
- `const Function&` – the [`Napi::Function`](function.md) to be called when the promise is fulfilled.
**Returns:** A new `Napi::Promise` that resolves or rejects based on the handler's result.
### Then
```cpp
Napi::Promise Napi::Promise::Then(napi_value onFulfilled, napi_value onRejected) const;
Napi::Promise Napi::Promise::Then(const Function& onFulfilled,
const Function& onRejected) const;
```
Attaches a fulfillment and rejection handlers to the promise and returns a new promise.
**Parameters:**
* `[in] onFulfilled`: The fulfillment handler for the promise. May be any of:
- `napi_value` – a JavaScript function to be called when the promise is fulfilled.
- `const Function&` – the [`Napi::Function`](function.md) to be called when the promise is fulfilled.
* `[in] onRejected` (optional): The rejection handler for the promise. May be any of:
- `napi_value` – a JavaScript function to be called when the promise is rejected.
- `const Function&` – the [`Napi::Function`](function.md) to be called when the promise is rejected.
### Catch
```cpp
Napi::Promise Napi::Promise::Catch(napi_value onRejected) const;
Napi::Promise Napi::Promise::Catch(const Function& onRejected) const;
```
Attaches a rejection handler to the promise and returns a new promise.
**Parameters:**
* `[in] onRejected`: The rejection handler for the promise. May be any of:
- `napi_value` – a JavaScript function to be called when the promise is rejected.
- `const Function&` – the [`Napi::Function`](function.md) to be called when the promise is rejected.
**Returns:** A new `Napi::Promise` that handles rejection cases.
[`Napi::Object`]: ./object.md
[`Napi::Function`]: ./function.md
|
