jerryscript/docs/12.EXT-REFERENCE-MODULE.md

Module API

This is a JerryScript extension that provides a means of loading modules. Fundamentally, a module is a name (stored as a string) that resolves to a jerry_value_t. This extension provides the function jerryx_module_resolve() which accepts the name of the module being requested as well as an array of so-called "resolvers" - functions which satisfy the signature jerryx_module_resolver_t. The resolvers in the list are called in sequence until one of them returns true and a jerry_value_t in its out parameter. The value is cached if it is not an error, so subsequent requests for the same name will not result in additional calls to the resolvers.

The purpose of having resolvers is to be able to account for the fact that different types of modules may be structured differently and thus, for each type of module a module resolver must be supplied at the point where an instance of that type of module is requested.

Additionally, this extension provides a means of easily defining so-called "native" JerryScript modules which can be resolved using the JerryScript native module resolver jerryx_module_native_resolver(), which can be passed to jerryx_module_resolve(). Note, however, that native JerryScript modules are only supported and jerryx_module_native_resolver() is only compiled in if compiler support for __attribute__ extensions is present. In effect this means that native JerryScript modules are available only when this extension is built with GCC or LLVM/clang. In the absence of such support, you may construct alternative module systems and provide your own resolver to jerryx_module_resolve().

jerryscript-ext/module.h defines the preprocessor directive JERRYX_NATIVE_MODULES_SUPPORTED only if support for native JerryScript modules is available.

jerryx_module_resolve

Summary

Load a copy of a module into the current context or return one that was already loaded if it is found.

Each function in resolvers_p will be called in sequence until one returns true and fills out its out-parameter with the jerry_value_t representing the requested module. If the jerry_value_t does not have the error flag set it will be cached. Thus, on subsequent calls with the same value for name, none of the functions in resolvers_p will be called.

Prototype

jerry_value_t
jerryx_module_resolve (const jerry_char_t *name,
                       jerryx_module_resolver_t *resolvers_p,
                       size_t resolver_count);

• name - the name of the module to load
• resolvers_p - the list of resolvers to call in sequence
• resolver_count - the number of resolvers in resolvers_p
• return value - jerry_value_t representing the module that was loaded, or the error that occurred in the process.

jerryx_module_native_resolver

Summary

The resolver for JerryScript modules. A pointer to this function can be passed in the second parameter to jerryx_module_resolve to search for the module among the JerryScript modules built into the binary. This function is available only if the preprocessor directive JERRYX_NATIVE_MODULES_SUPPORTED is defined.

Prototype

bool
jerryx_module_native_resolver (const jerry_char_t *name,
                               jerry_value_t *result)

• name - the name of the module to find
• result - out - place where to store the resulting module instance
• return value - true if the module was found and stored in result, and false otherwise

Module data types

jerryx_native_module_on_resolve_t

Summary

Function pointer type for a function that will create an instance of a native module. This type is only defined if the preprocessor directive JERRYX_NATIVE_MODULES_SUPPORTED is defined.

Prototype

typedef jerry_value_t (*jerryx_native_module_on_resolve_t) (void);

jerryx_module_resolver_t

Summary

Function pointer type for a module resolver

Prototype

typedef bool (*jerryx_module_resolver_t) (const jerry_char_t *name, jerry_value_t *result);

Example

bool
load_and_evaluate_js_file (const jerry_char_t *name, jerry_value_t *result)
{
  bool return_value = false;
  char *js_file_contents = NULL;
  int file_size = 0;
  FILE *js_file = fopen (name, "r");

  if (js_file)
  {
    /* We have successfully opened the file. Now, we establish its size. */
    file_size = fseek (js_file, 0, SEEK_END);
    fseek (js_file, 0, SEEK_SET);

    /* We allocate enough memory to store the contents of the file. */
    js_file_contents = malloc (file_size);
    if (js_file_contents)
    {
      /* We read the file into memory and call jerry_eval (), assigning the result to the out-parameter. */
      fread (js_file_contents, file_size, 1, js_file);
      (*result) = jerry_eval (js_file_contents, file_size, false);

      /* We release the memory holding the contents of the file. */
      free (js_file_contents);
      return_value = true;
    }

    /* We close the file. */
    fclose (js_file);
  }

  return return_value;
}

We can now load JavaScript files:

static const jerryx_module_resolver_t resolvers =
{
  /* Consult the JerryScript module resolver first, in case the requested module is a compiled-in JerryScript module. */
  jerryx_module_native_resolver,

  /*
   * If the requested module is not a JerryScript module, assume it is a JavaScript file on disk and use the above-
   * defined JavaScript file loader to load it.
   */
  load_and_evaluate_js_file
};
jerry_value_t js_module = jerryx_module_resolve (requested_module, resolvers, 2);

Module helper macros

JERRYX_NATIVE_MODULE

Summary

Helper macro to define a JerryScript module. Currently stores the name of the module and its initializer in an executable linker section. This macro is available only if the preprocessor directive JERRYX_NATIVE_MODULES_SUPPORTED is defined.

Note: The helper macro must appear at the bottom of a source file, and no semicolon must follow it.

Prototype

#define JERRYX_NATIVE_MODULE(module_name, on_resolve_cb)

• module_name - the name of the module without quotes
• on_resolve_cb - the function of type jerryx_native_module_on_resolve_t that will be called when the module needs to be loaded.

Example

#include "jerryscript.h"
#include "jerryscript-ext/module.h"

static jerry_value_t
my_module_on_resolve (void)
{
  return jerry_create_external_function (very_useful_function);
} /* my_module_on_resolve */

/* Note that there is no semicolon at the end of the next line. This is how it must be. */
JERRYX_NATIVE_MODULE (my_module, my_module_on_resolve)