"Define a TypeScript function type that can return either an object or a string depending on whether it succeeds or fails

Question

"Define a TypeScript function type that can return either an object or a string depending on whether it succeeds or fails

I encountered an issue with a function that either returns a Promise on success or a string on error.

async create(createDebtorDto: CreateDebtorDto): Promise<Debtor> {
    console.log("createDebtorDto", createDebtorDto)
    try{
      const createdDebtor = new this.debtorModel(createDebtorDto);
      return await createdDebtor.save();
    }catch(err){
      console.log('err', err)
        return `Unable to create debtor ${err}`;
    }
  }

During the transpilation process, I encountered this error message:

src/debtors/debtors.service.ts(19,5): error TS2322: Type 'string' is not assignable to type 'Debtor'.

This situation made me question whether it's possible to define a function that returns a Promise in case of success and a String when encountering an error without compromising the benefits of static typing.

Is it feasible to have a function that returns a promise on success and a string on error?

typescript

Answer 1

Answer №1

One way to approach this situation is by specifying that the output could potentially be any of those two types:

: Promise<Debtor | string>

However, it's more common for individuals to anticipate handling errors properly:

async create(createDebtorDto: CreateDebtorDto): Promise<Debtor> {
    console.log("createDebtorDto", createDebtorDto)
    try {
        const createdDebtor = new this.debtorModel(createDebtorDto);
        return await createdDebtor.save();
    } catch (err) {
        console.log('err', err)
        throw `Unable to create debtor ${err}`;
    }
}

This method should be invoked in the following manner:

myInstance.create(dto).then((debtor) => {
    // perform actions with debtor...
}).catch((error) => {
    alert(error);
});

By adhering to this practice, there is no need to verify the type of debtor within the resolve handler - thereby avoiding unexpected outcomes for individuals accustomed to working with asynchronous operations and promises.

Answer 2

One way to approach this situation is by specifying that the output could potentially be any of those two types:

: Promise<Debtor | string>

However, it's more common for individuals to anticipate handling errors properly:

async create(createDebtorDto: CreateDebtorDto): Promise<Debtor> {
    console.log("createDebtorDto", createDebtorDto)
    try {
        const createdDebtor = new this.debtorModel(createDebtorDto);
        return await createdDebtor.save();
    } catch (err) {
        console.log('err', err)
        throw `Unable to create debtor ${err}`;
    }
}

This method should be invoked in the following manner:

myInstance.create(dto).then((debtor) => {
    // perform actions with debtor...
}).catch((error) => {
    alert(error);
});

By adhering to this practice, there is no need to verify the type of debtor within the resolve handler - thereby avoiding unexpected outcomes for individuals accustomed to working with asynchronous operations and promises.

Answer 3

Answer №2

It is important to note that your function always returns a Promise due to the presence of the catch statement following an await, which makes it dependent on an asynchronous value. Additionally, async methods are required to return a promise. Therefore, you should ensure that your function returns a Promise<Debtor| string> as the result of the async operation can either be a string (in case of an error) or a Debtor in case of success.

async create(createDebtorDto: CreateDebtorDto): Promise<Debtor| string> {
    console.log("createDebtorDto", createDebtorDto)
    try{
        const createdDebtor = new this.debtorModel(createDebtorDto);
        return await createdDebtor.save();
    }catch(err){
        console.log('err', err)
        return `Unable to create debtor ${err}`;
    }
}

Note: It may not be ideal design to return a string in case of an error. Consider throwing an exception and catching it in the caller instead.

async create(createDebtorDto: CreateDebtorDto): Promise<Debtor> {
    console.log("createDebtorDto", createDebtorDto)
    try{
        const createdDebtor = new this.debtorModel(createDebtorDto);
        return await createdDebtor.save();
    }catch(err){
        console.log('err', err)
        throw `Unable to create debtor ${err}`;
    }
}

Answer 4

It is important to note that your function always returns a Promise due to the presence of the catch statement following an await, which makes it dependent on an asynchronous value. Additionally, async methods are required to return a promise. Therefore, you should ensure that your function returns a Promise<Debtor| string> as the result of the async operation can either be a string (in case of an error) or a Debtor in case of success.

async create(createDebtorDto: CreateDebtorDto): Promise<Debtor| string> {
    console.log("createDebtorDto", createDebtorDto)
    try{
        const createdDebtor = new this.debtorModel(createDebtorDto);
        return await createdDebtor.save();
    }catch(err){
        console.log('err', err)
        return `Unable to create debtor ${err}`;
    }
}

Note: It may not be ideal design to return a string in case of an error. Consider throwing an exception and catching it in the caller instead.

async create(createDebtorDto: CreateDebtorDto): Promise<Debtor> {
    console.log("createDebtorDto", createDebtorDto)
    try{
        const createdDebtor = new this.debtorModel(createDebtorDto);
        return await createdDebtor.save();
    }catch(err){
        console.log('err', err)
        throw `Unable to create debtor ${err}`;
    }
}

"Define a TypeScript function type that can return either an object or a string depending on whether it succeeds or fails

Answer №1

Answer №2

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