Typescript TS 推断从不类型，因为它无法在 forEach 循环中进行赋值

错误读数为Property 'id' does not exist on type 'never'.

我对TypeScript了解得足够多，以至于我认为Control Flow Analysis会让我在这个场景中被涵盖.当我宣布:

let quantityRange: QuantityLevels | false = false;

并且，在迭代其中一个函数参数时，可以分配QuantityLevels类型的变量may:

priceList.quantityLevels.forEach((ql) => {
  if (ql.min <= quantity && ql.max >= quantity) {
    quantityRange = ql;
  }
});

在这种情况下，我们通过抛出一个错误，消除了函数剩余部分的赋值为quantityRange的可能性.然而，TypeScript推断变量quantityRange只能被赋值为false，对于函数的其余部分，它被键入为never.

当我try 使用quantityRange的属性时，我被拒绝了:

const priceByProduct = priceList.prices[product.id][quantityRange.id].price;
// error: Property 'id' does not exist on type 'never'.

这是怎么回事？如果我分配任务

quantityRange = priceList.quantityLevels[0];

(priceList.quantityLevels被打成Array<QuantityLevels>)，那么一切都好了.这让我想到，当谈到TypeScript处理函数/块范围变量的方式时，我必须有一个不完整的理解模型.我通读了打字本手册中的what I think个是relevant sections，结果空手而来(或者可能是空着脑袋).

该代码不会通过build次操作，但在开发和测试环境中，它的工作方式与预期一致.

The code below also rendered in TypeScript playground, along with errors.

function getPrice({ variantID, quantity, priceList }: GetPriceProps) {
  let quantityRange: QuantityLevels | false = false;

  priceList.quantityLevels.forEach((ql) => {
    if (ql.min <= quantity && ql.max >= quantity) {
      quantityRange = ql;
    }
  });

  // quantityRange = priceList.quantityLevels[0];
  // ^ Manually assigning this (which is what's happening in the above for loop) 
  //   makes TS understand.

  if (!quantityRange) {
    throw new Error(`No quantity range found for ${quantity} of ${variantID}.`);
  }

  const product = priceList.targets.find(
    (t) => t.variants.find((v) => v.id === variantID),
  );

  if (!product) {
    throw new Error(`Could not find a product related to variant ${variantID}.`);
  }

  const priceByProduct = priceList.prices[product.id][quantityRange.id].price;
  const priceByVariant = priceList.prices[product.id][quantityRange.id][variantID];

  return (priceByVariant || priceByProduct);
};

type QuantityLevels = {
  id: string;
  min: number;
  max: number;
}

interface NamedEntity {
  id: string;
  name: string; 
};

interface Product extends NamedEntity {
  variants: Array<NamedEntity>;
}

type PriceList = {
  targets: Array<Product>;
  quantityLevels: Array<QuantityLevels>;
  prices: {
    [productID: string]: {
      [quantityLevelID: string]: {
        price: number;
        [variantID: string]: number;
      }
    }
  }
};

type GetPriceProps = {
  variantID: string;
  quantity: number;
  priceList: PriceList;
};