泛型

function identity<Type>(arg: Type): Type {
  return arg;
}

// 箭头函数的泛型必须有extends，否则语法无法识别。
const identity =<T extends {}>(arg: T): T => {
  return arg;
};

interface Length{
  length:number
}
function  fn<T extends Length>(foo:T):T{
  console.log(foo.length)
  return foo
}

fn(1) //error:类型“number”的参数不能赋给类型“Length”的参数。
fn([1,2])

const test = forwardRef(
    <D extends {}>(
        { fieldItems, formOption, isEdit, defaultValue, style, onChange }: IFormProp<D>,
        ref: React.Ref<IFormBag<D>>
    ) => {
        return null;
    }
);