TypeScript中Promise<void>无法赋值给Promise<transactionArgument>报错排查

我在TypeScript中编写了事务处理代码，出现编译错误：Type 'Promise<void>' is not assignable to type 'Promise<transactionArgument>'。我认为函数已经返回了类型为transactionArgument的transaction变量，想请教报错原因。

代码示例 #

type RegisterPricingTransactionInfo = (pricingTransaction: transactionArgument, createdAt: Date, createdBy: string, datasource: any) => Promise<transactionArgument>

export const registerPricingTransactionInfo: RegisterPricingTransactionInfo = async (pricingTransaction, createdAt, createdBy, datasource) => {
    let urlCode;
    let transaction: transactionArgument;
    transaction = { test: "test" }

    await datasource.manager.transaction(async (transactionalEntityManager: EntityManager) => {
        try {
            await transactionalEntityManager.save(transaction)
            return transaction;
        } catch (error) {
            console.error(error)
        }
    })
}

错误信息 #

"message": "Type '(pricingTransaction: transactionArgument, createdAt: Date, createdBy: string, datasource: any) => Promise<void>' is not assignable to type 'RegisterPricingTransactionInfo'.
  Type 'Promise<void>' is not assignable to type 'Promise<transactionArgument>'.
    Type 'void' is not assignable to type 'transactionArgument'.",

报错原因与修复方案 #

核心原因 #

你外层的registerPricingTransactionInfo函数没有返回任何值：虽然你在transaction的回调函数里返回了transaction，但这个返回值属于回调函数本身，不会自动成为外层函数的返回值。外层函数执行完await datasource.manager.transaction(...)后就直接结束，没有明确的返回语句，所以TypeScript推断它的返回类型是Promise<void>，和你定义的Promise<transactionArgument>类型不匹配。

另外，即使回调里有返回值，如果catch分支没有处理返回值或抛出错误，回调函数的返回类型会变成Promise<void>，同样会导致外层类型不匹配。

修复后的代码 #

type RegisterPricingTransactionInfo = (pricingTransaction: transactionArgument, createdAt: Date, createdBy: string, datasource: any) => Promise<transactionArgument>

export const registerPricingTransactionInfo: RegisterPricingTransactionInfo = async (pricingTransaction, createdAt, createdBy, datasource) => {
    let transaction: transactionArgument = { test: "test" };

    // 接收回调的返回值，并作为外层函数的返回值
    return await datasource.manager.transaction(async (transactionalEntityManager: EntityManager) => {
        try {
            await transactionalEntityManager.save(transaction);
            return transaction;
        } catch (error) {
            console.error(error);
            // 重新抛出错误，让外层Promise进入reject状态，符合类型要求
            throw error;
        }
    });
}

关键修复点 #

1. 在外层函数中添加return，将transaction方法的执行结果（即回调的返回值）作为外层函数的返回值。
2. 在catch分支中重新抛出错误：如果只捕获错误不处理，回调函数会返回Promise<void>，外层函数即使return了这个结果，类型依然不匹配。重新抛出错误后，外层Promise会被标记为reject，符合Promise<transactionArgument>的类型定义（Promise要么resolve为transactionArgument，要么reject）。

内容的提问来源于stack exchange，提问作者Heisenberg