Encountering a "Property does not exist on type" error when transitioning from JavaScript to TypeScript

Greetings, I recently made the transition from JS to TS for improved coding practices. However, TypeScript is throwing me these errors- the Validate function worked flawlessly in JavaScript.

Property 'user' does not exist on type '{ isValid: boolean; user: string; userLowercase: string; } | undefined'
Property 'userLowercase' does not exist on type '{ isValid: boolean; user: string; userLowercase: string; } | undefined'

Here are the relevant files:

//functions.ts
function Validate(arg) {
    if (!arg) return
    let query :string = arg.toString().replace(""@"", "")
    let regex = /^[a-zA-Z0-9._-]+$/
    let isValid = regex.test(query)
    if (!isValid) return
    let userLowercase :string = query.toLowerCase()
    return { isValid: true, user : query, userLowercase }
}

//firstCommand.ts 
import { Validate } from './functions'

class firstCommand extends Commands {
    async command() {
        if (!Validate(this.arg)) return
        const { user, userLowercase } = Validate(this.arg)
        ///
    }
}

I have tried searching on Google but couldn't find anyone with a similar issue. Am I possibly writing the code incorrectly? Any guidance on how to resolve this would be highly appreciated.

Thank you in advance!

Answer №1

If you encounter this error, it is indicating the issue at hand:

The property 'user' does not exist on the type
  '{ isValid: boolean; user: string; userLowercase: string; } | undefined'

Take note of the last part | undefined. This signifies that the type of your variable could either be the expected value or possibly undefined. If it happens to actually be undefined, then your program will encounter a crash at this point.

The root of the problem lies within your Validate() function. Pay attention to these lines:

if (!arg) return

And this line:

if (!isValid) return

Therefore, if arg or isValid are falsy, the function will return undefined. It only returns the complete object if it manages to pass through those initial return statements.

This implies that the return type of your function is as follows:

{ isValid: boolean; user: string; userLowercase: string; } | undefined

To resolve this issue, you need to verify the existence of the value before assuming its presence.

const validationResult = Validate(this.arg)
if (validationResult) {
  const { user, userLowercase } = validationResult
  // continue...
}

Typescript will acknowledge that you have checked for the variable's existence and allow you to proceed from within that if statement.

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