Arrow function used in the built-in findIndex function

I am new to javascript and doing a nested find in an array as follow:

    var target = this.newRubric.categories
    .find((category) => category.name == this.indexTracker.selectedCategory)
    .criteria.find(
      (criterion) => criterion.name == this.indexTracker.selectedCriterion
    )
    .descriptors.findIndex(
      (descriptor) =>
        //Error here, works by removing the curly brackets 
        {descriptor.name == this.indexTracker.selectedDescriptor}
    );
  console.log(target);

It always returned -1 which is not found until I removed the curly braces, I know that if there was only one line of code the curly braces is not necessary, but why did adding it will cause an error in the findIndex method?