typescript type is not assignable to type `symbol` 错误

Question

export type anchorPositionType = "middle" | "left" | "right" | "top" | "bottom" | customAnchorType;
export type customAnchorType = { rightness?: number; bottomness?: number };

const getAnchorOffset = (width:number, height:number): { [key in anchorPositionType]: customAnchorType } => // here 'anchorPositionType' getting the error Type 'customAnchorType' is not assignable to type 'symbol' and i don't understand why?
 {
   return {
     middle: { rightness: width * 0.5, bottomness: height * 0.5 },
     left: { rightness: 0, bottomness: height * 0.5 },
     right: { rightness: width, bottomness: height * 0.5 },
     top: { rightness: width * 0.5, bottomness: 0 },
     bottom: { rightness: width, bottomness: height * 0.5 }
   };
};

我不知道是什么问题。为什么函数签名处的 anchorPositionType 出现错误 Type 'customAnchorType' is not assignable to type 'symbol'？

Answer 1

好吧，在我的例子中，编译器提示 customAnchorType 不是联合类型，因此不能对他使用 in 运算符。

type anchorPositionType = 'middle' | 'left' | 'right' | 'top' | 'bottom' | customAnchorType;
type customAnchorType = { rightness?: number; bottomness?: number };
type test = { [key in anchorPositionType]: customAnchorType };
                      ^
    Type 'customAnchorType' is not assignable to type 'symbol'. 
    
    ('customAnchorType' is an object) 

如果我们要从 anchorPositionType 中移除 customAnchorType:

type anchorPositionType = 'middle' | 'left' | 'right' | 'top' | 'bottom' 

错误会消失，因为现在 anchorPositionType 是 middle' | '左' | '正确' | '顶部' | '底部