Angular's annotation function

angular.annotate.js

function annotate(fn, strictDi, name) {
  var $inject,
      fnText,
      argDecl,
      last;

  if (typeof fn === 'function') {
    if (!($inject = fn.$inject)) {
      $inject = [];
      if (fn.length) {
        if (strictDi) {
          if (!isString(name) || !name) {
            name = fn.name || anonFn(fn);
          }
          throw $injectorMinErr('strictdi',
            '{0} is not using explicit annotation and cannot be invoked in strict mode', name);
        }
        fnText = fn.toString().replace(STRIP_COMMENTS, '');
        argDecl = fnText.match(FN_ARGS);
        forEach(argDecl[1].split(FN_ARG_SPLIT), function(arg){
          arg.replace(FN_ARG, function(all, underscore, name){
            $inject.push(name);
          });
        });
      }
      fn.$inject = $inject;
    }
  } else if (isArray(fn)) {
    last = fn.length - 1;
    assertArgFn(fn[last], 'fn');
    $inject = fn.slice(0, last);
  } else {
    assertArgFn(fn, 'fn', true);
  }
  return $inject;
}

For better understanding of err.handling:

function assertArg(arg, name, reason) {
  if (!arg) {
    throw ngMinErr('areq', 'Argument \'{0}\' is {1}', (name || '?'), (reason || 'required'));
  }
  return arg;
}

function assertArgFn(arg, name, acceptArrayAnnotation) {
  if (acceptArrayAnnotation && isArray(arg)) {
      arg = arg[arg.length - 1];
  }

  assertArg(isFunction(arg), name, 'not a function, got ' +
      (arg && typeof arg === 'object' ? arg.constructor.name || 'Object' : typeof arg));
  return arg;
}