# Creating and Calling JS Functions Using JSVM-API

## Introduction

JSVM-API provides APIs for calling JavaScript (JS) functions and passing parameters or creating JS methods in C/C++.

## Basic Concepts

Functions are blocks of reusable code that performs specific tasks or operations. You can define functions to implement different operations. Functions provide a way to modularize and structure code, helping make your code more organized, reusable, and maintainable.

## Available APIs

| API                      | Description                      |
|----------------------------|--------------------------------|
| OH_JSVM_GetCbInfo          | Obtains detailed information about the call, such as the parameters and **this** pointer, from the given callback information.|
| OH_JSVM_CallFunction       | Calls a JS function from a C/C++ addon.|
| OH_JSVM_IsFunction         | Checks whether an object is a function object.|
| OH_JSVM_CreateFunction     | Creates a JS function to call a C/C++ function in JS environment. The function can be called only after it is set in a JS object.|

## Example

If you are just starting out with JSVM-API, see [JSVM-API Development Process](use-jsvm-process.md). The following demonstrates only the C++ code involved in creating and calling functions.

### OH_JSVM function Test

Full code for the CPP test. The entry is **TEST_FUNC**.
``` cpp
#include "hilog/log.h"
#include "ark_runtime/jsvm.h"

#define LOG_DOMAIN 0x3200
#define LOG_TAG "APP"

#define CHECK_RET(cond) \
  if ((cond)) { \
    const JSVM_ExtendedErrorInfo* info; \
    OH_JSVM_GetLastErrorInfo(env, &info); \
    OH_LOG_ERROR(LOG_APP, "jsvm fail file: %{public}s line: %{public}d ret = %{public}d message = %{public}s", __FILE__, __LINE__, cond, info != nullptr ? info->errorMessage : ""); \
    return -1;   \
  }

#define CHECK(cond) \
  if (!(cond)) { \
     OH_LOG_ERROR(LOG_APP, "jsvm fail file: %{public}s line: %{public}d ret = %{public}d", __FILE__, __LINE__, cond); \
     return -1;   \
  }

JSVM_Value NativeCreateFunctionTest(JSVM_Env env, JSVM_CallbackInfo info) {
    void *data;
    size_t argc = 1;
    JSVM_Value argv[1] = {nullptr};
    JSVM_Value thisArg;
    // Obtain the callback parameters.
    JSVM_Status ret = OH_JSVM_GetCbInfo(env, info, &argc, &argv[0], &thisArg, &data);
    if (ret != JSVM_OK) {
      const JSVM_ExtendedErrorInfo* info;
      OH_JSVM_GetLastErrorInfo(env, &info);
      OH_LOG_ERROR(LOG_APP, "jsvm fail file: %{public}s line: %{public}d ret = %{public}d message = %{public}s", __FILE__, __LINE__, ret, info != nullptr ? info->errorMessage : "");
      return nullptr;
    }
    char message[256];
    OH_JSVM_GetValueStringLatin1(env, argv[0], message, 256, nullptr);
    if (data == nullptr) {
        OH_LOG_ERROR(LOG_APP, "jsvm: %{public}s; callback data null", message);
    } else {
        OH_LOG_INFO(LOG_APP, "jsvm: %{public}s; %{public}s", message, (char*)data);
    }
    return nullptr;
}

static int32_t TEST_FUNC() {
    JSVM_InitOptions initOptions{};
    JSVM_VM vm;
    JSVM_Env env = nullptr;
    JSVM_VMScope vmScope;
    JSVM_EnvScope envScope;
    JSVM_HandleScope handleScope;
    JSVM_Value result;
    static bool isVMInit = false;
    if (!isVMInit) {
        isVMInit = true;
        // A single process needs to be initialized only once.
        OH_JSVM_Init(&initOptions);
    }
    CHECK_RET(OH_JSVM_CreateVM(nullptr, &vm));
    CHECK_RET(OH_JSVM_CreateEnv(vm, 0, nullptr, &env));
    CHECK_RET(OH_JSVM_OpenVMScope(vm, &vmScope));
    CHECK_RET(OH_JSVM_OpenEnvScope(env, &envScope));
    CHECK_RET(OH_JSVM_OpenHandleScope(env, &handleScope));

    // Create a function and check it.
    char hello[] = "Hello World!";
    JSVM_CallbackStruct cb = {NativeCreateFunctionTest, (void*)hello};
    JSVM_Value func;
    CHECK_RET(OH_JSVM_CreateFunction(env, "", JSVM_AUTO_LENGTH, &cb, &func));
    bool isFunction = false;
    CHECK_RET(OH_JSVM_IsFunction(env, func, &isFunction));
    CHECK(isFunction);

    // Set the function to a global object.
    JSVM_Value global;
    CHECK_RET(OH_JSVM_GetGlobal(env, &global));
    JSVM_Value key;
    CHECK_RET(OH_JSVM_CreateStringUtf8(env, "NativeFunc", JSVM_AUTO_LENGTH, &key));
    CHECK_RET(OH_JSVM_SetProperty(env, global, key, func));

    // Call the function through OH_JSVM_CallFunction.
    JSVM_Value argv[1] = {nullptr};
    OH_JSVM_CreateStringUtf8(env, "jsvm api call funtion", JSVM_AUTO_LENGTH, &argv[0]);
    CHECK_RET(OH_JSVM_CallFunction(env, global, func, 1, argv, &result));

    // Call the function through the script.
    JSVM_Script script;
    JSVM_Value jsSrc;
    const char* srcCallNative = R"JS(NativeFunc('js source call funtion');)JS";
    CHECK_RET(OH_JSVM_CreateStringUtf8(env, srcCallNative, JSVM_AUTO_LENGTH, &jsSrc));
    CHECK_RET(OH_JSVM_CompileScript(env, jsSrc, nullptr, 0, true, nullptr, &script));
    CHECK_RET(OH_JSVM_RunScript(env, script, &result));

    CHECK_RET(OH_JSVM_CloseHandleScope(env, handleScope));
    CHECK_RET(OH_JSVM_CloseEnvScope(env, envScope));
    CHECK_RET(OH_JSVM_CloseVMScope(vm, vmScope));
    CHECK_RET(OH_JSVM_DestroyEnv(env));
    CHECK_RET(OH_JSVM_DestroyVM(vm));
    return 0;
}
```
**Expected output**
```
jsvm: jsvm api call funtion; Hello World!
jsvm: js source call funtion; Hello World!
```