Question

当我们使用map时，字符串文字联合类型失败，而当我们直接创建数组时失败。

type Unit = 'g' | 'ml';

class Measurement { 
    unit: Unit;
}

class Thing { 
    measurements: Measurement[];
}

// works
const things: Thing[] = [{
    measurements: [{
        unit: 'g'
    }]
}];

// fails
const thingsToo: Thing[] = Array.from({ length: 5 }).map((i) => ({
    measurements: [{
        unit: 'g'
    }]
}));

错误：

Type '{ measurements: { unit: string; }[]; }[]' is not assignable to type 'Thing[]'.
  Type '{ measurements: { unit: string; }[]; }' is not assignable to type 'Thing'.
    Types of property 'measurements' are incompatible.
      Type '{ unit: string; }[]' is not assignable to type 'Measurement[]'.
        Type '{ unit: string; }' is not assignable to type 'Measurement'.
          Types of property 'unit' are incompatible.
            Type 'string' is not assignable to type 'Unit'.

为什么在map期间会发生这种情况？我们如何预防呢？

一种方法是强制转换为Measurement。虽然尚不清楚为什么这样做是必需的，而且比不强制转换更冗长，所以我们宁愿不使用强制转换，除非那是唯一的方法。

// works
const thingsToo: Thing[] = Array.from({ length: 5 }).map((i) => ({
    measurements: [{
        unit: 'g'
    } as Measurement]
}));

Answer 1

It's explained here

字符串文字类型的工作方式是字符串文字表达式必须由字符串文字类型（或并集）在上下文中键入包含字符串文字类型）以将check输入为字符串文字类型而不是字符串。

在您的代码中，'g'字符串文字类型被扩展为string，因为没有上下文可以将其保留为Array...map表达式中的文字。

另一种解决方法是为map回调添加显式返回类型：

type Unit = 'g' | 'ml';

class Measurement { 
    unit: Unit;
}

class Thing { 
    measurements: Measurement[];
}

// works
const things: Thing[] = [{
    measurements: [{
        unit: 'g'
    }]
}];

// works
const thingsToo: Thing[] = Array.from({ length: 5 }).map((i): Thing => ({
    measurements: [{
        unit: 'g'
    }]
}));

字符串文字联合类型在映射操作期间创建时无法编译

1 个答案: