duktape/website/api/duk_pcall.yaml

name: duk_pcall

proto: |
  duk_int_t duk_pcall(duk_context *ctx, duk_idx_t nargs);

stack: |
  [ ... func! arg1! ...! argN! ] -> [ ... retval! ]  (if success, return value == 0)
  [ ... func! arg1! ...! argN! ] -> [ ... err! ]  (if failure, return value != 0)

summary: |
  <p>Call target function <code>func</code> with <code>nargs</code> arguments
  (not counting the function itself).  The function and its arguments
  are replaced by a single return value or a single error value.
  An error thrown during the function call is caught.</p>

  <p>The return value is:</p>
  <ul>
  <li><code>DUK_EXEC_SUCCESS</code> (0): call succeeded, <code>nargs</code> arguments are replaced
      with a single return value.  (This return code constant is guaranteed to be zero, so
      that one can check for success with a "zero or non-zero" check.)</li>
  <li><code>DUK_EXEC_ERROR</code>: call failed, <code>nargs</code> arguments are replaced with a
      single error value.  (In exceptional cases, e.g. when there are too few
      arguments on the value stack, the call returns non-zero but may leave the stack
      in an inconsistent state.)</li>
  </ul>

  <div class="note">
  Unlike most Duktape API calls, this call returns zero on success.  This allows
  multiple error codes to be defined later.
  </div>

  <p>Error objects caught are typically instances of Error and have useful
  properties like <code>.stack</code>, <code>.fileName</code>, and
  <code>.lineNumber</code>.  These can be accessed using the normal property
  methods.  However, arbitrary values can be thrown so you should avoid
  assuming that's always the case.</p>

  <p>The target function <code>this</code> binding is initially set to
  <code>undefined</code>.  If the target function is not strict, the binding
  is replaced by the global object before the function is invoked; see
  <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-10.4.3">Entering Function Code</a>.
  If you want to control the <code>this</code> binding, you can use
  <code><a href="#duk_pcall_method">duk_pcall_method()</a></code> or
  <code><a href="#duk_pcall_prop">duk_pcall_prop()</a></code> instead.</p>

example: |
  /* Assume target function is already on stack at func_idx; the target
   * function adds arguments and returns the result.
   */

  duk_idx_t func_idx;
  duk_int_t rc;

  /* Basic example: function call and duk_safe_to_string() error print. */

  duk_dup(ctx, func_idx);
  duk_push_int(ctx, 2);
  duk_push_int(ctx, 3);
  rc = duk_pcall(ctx, 2);  /* [ ... func 2 3 ] -> [ 5 ] */
  if (rc == DUK_EXEC_SUCCESS) {
      printf("2+3=%ld\n", (long) duk_get_int(ctx, -1));
  } else {
      /* Coercing with duk_safe_to_string() is a useful default, but you can
       * also look up e.g. the .stack property of the error.
       */
      printf("error: %s\n", duk_safe_to_string(ctx, -1));
  }
  duk_pop(ctx);

  /* Accessing .stack to print a stack trace if the value caught is an
   * Error instance.
   */

  duk_dup(ctx, func_idx);
  duk_push_int(ctx, 2);
  duk_push_int(ctx, 3);
  rc = duk_pcall(ctx, 2);  /* [ ... func 2 3 ] -> [ 5 ] */
  if (rc == DUK_EXEC_SUCCESS) {
      printf("2+3=%ld\n", (long) duk_get_int(ctx, -1));
  } else {
      if (duk_is_error(ctx, -1)) {
          /* Accessing .stack might cause an error to be thrown, so wrap this
           * access in a duk_safe_call() if it matters.
           */
          duk_get_prop_string(ctx, -1, "stack");
          printf("error: %s\n", duk_safe_to_string(ctx, -1));
          duk_pop(ctx);
      } else {
          /* Non-Error value, coerce safely to string. */
          printf("error: %s\n", duk_safe_to_string(ctx, -1));
      }
  }
  duk_pop(ctx);

tags:
  - call
  - protected

seealso:
  - duk_pcall_method
  - duk_pcall_prop

introduced: 1.0.0