code: function assertArrayOf(elementValidatorFn, value, name, description) {
    if (!underscore.isArray(value)) {
      throw new Error(
          '<' + value + '> is not an array, ' + name + ': ' + description);
    }
  
    // Check all elements.
    value.forEach(function(element, index) {
      elementValidatorFn(element, name + '[' + index + ']', description);
    });
  }
docs: ============================================================================== Object & Array Validators ============================================================================== 
  Throws an Error if value isn't an array, and invokes elementValidatorFn for
  each element of the array.
  @param {function(*, string, string)} elementValidatorFn
  @param {*} value
  @param {string} name The option name.
  @param {string} description Description of the option.