Ways to convert a single string or array of strings into an array of strings

Question

Ways to convert a single string or array of strings into an array of strings

I am encountering a situation where I receive a response from Node.js that includes multiple data types such as

string | QueryString.ParsedQs | string[] | QueryString.ParsedQs[]

. However, I specifically require it to be only of type string[]. How can I achieve this in TypeScript?

To provide some context: the challenge arises because Node.js returns

string | QueryString.ParsedQs | string[] | QueryString.ParsedQs[]

when accessing req.query, but it is crucial for me to have it as a string[], especially when using TypeORM with a where-in clause.

Below is the code snippet I have tried so far, which unfortunately does not yield the desired outcome:

import { In } from "typeorm";    
const { investmentIds } = req.query;
const historicalReturns = await historicalReturnRepository.find({
    where: In(investmentIds)
});

typescript

Answer 1

Answer №1

When you assert req.query to be a string[], it doesn't actually change its underlying type - it just helps TypeScript understand how to handle it during type checking. However, using assertions without a clear reason can lead to potential errors. Some people mistakenly believe that assertions alter the actual object type, but this is not the case.

If you want to ensure that your processing code receives an array, you can do something like this:

const { dataIds } = req.query
const dataArray = Array.isArray(dataIds) ? dataIds : [dataIds];

You can now confidently use dataArray as an array in your code.

Answer 2

When you assert req.query to be a string[], it doesn't actually change its underlying type - it just helps TypeScript understand how to handle it during type checking. However, using assertions without a clear reason can lead to potential errors. Some people mistakenly believe that assertions alter the actual object type, but this is not the case.

If you want to ensure that your processing code receives an array, you can do something like this:

const { dataIds } = req.query
const dataArray = Array.isArray(dataIds) ? dataIds : [dataIds];

You can now confidently use dataArray as an array in your code.

Ways to convert a single string or array of strings into an array of strings

Answer №1

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