code: function transformModel (options, data) {
    const prop = (options.model && options.model.prop) || 'value';
    const event = (options.model && options.model.event) || 'input'
    ;(data.attrs || (data.attrs = {}))[prop] = data.model.value;
    const on = data.on || (data.on = {});
    const existing = on[event];
    const callback = data.model.callback;
    if (isDef(existing)) {
      if (
        Array.isArray(existing)
          ? existing.indexOf(callback) === -1
          : existing !== callback
      ) {
        on[event] = [callback].concat(existing);
      }
    } else {
      on[event] = callback;
    }
  }
docs: transform component v-model info (value and callback) into prop and event handler respectively.