Question: Why use .catch on the promise

[terenced]

I am curious why this change was introduced.

// Original implementation  
export const until = async <DataType = unknown, ErrorType = Error>(promise: () => Promise<DataType>): Promise<[ErrorType, DataType]> => {
  try {
    const data = await promise()
    return [null, data]
  } catch (error) {
    return [error, null]
}
// Current implementation 
export const until = async <DataType = unknown, ErrorType = Error>(promise: () => Promise<DataType>): Promise<[ErrorType, DataType]> => {
  try {
   const data = await promise().catch((error) => { // .catch added
      throw error
    })
    return [null, data]
  } catch (error) {
    return [error, null]
}

Is there a benefit of calling .catch on the promise? It is my understanding that the catch block would handle throws?

[newpost]

@terenced can you tell me why?thank you

[kettanaito]

The .catch() block was added to propagate unhandled exceptions to the library.

When there's an exception thrown during a Promise, it will not be caught by a try/catch block around it:

try { new Promise(() => { throw new Error('Hey') }) } catch (e) { console.log(e) }

The console.log will never be called. The thrown exception will be printed as an UnhandledPromiseRejectionWarning.

Flow-wise, that is still an unsuccessful promise execution, so this library should execute the return [error, null] closure for the dependent code to handle it like an error. Adding an explicit .catch() statement to the given promise catches any errors occurring in that promise.

This solution also doesn't interfere with you defining your custom .catch(), as catch doesn't have an accumulative nature. If you've defined your custom .catch() block, it will get executed first, and the library's .catch() will not, unless you throw another exception in your own catch block.

new Promise(() => { throw new Error('Hey') }).catch(e => console.log('Custom', e)).catch(e => console.log('Library', e))

[terenced]

@kettanaito Thank you!

[sbking]

@kettanaito I think this might be incorrect - await should throw on promise rejection

In your example you're just missing an await - add it and console.log is called:

try { await new Promise(() => { throw new Error('Hey') }) } catch (e) { console.log(e) }