Discuss
  • Default `this` value for static promise functions

Default `this` value for static promise functions

# a.d.bergi at web.de (10 years ago)

I've just got bitten by

return getError().then(Promise.reject);

where reject threw an error that its this value was not a Promise constructor.

Would it help to make the C constructor (in Promise.resolve, Promise.reject, Promise.all, Promise.race algorithms) default to Promise when no this value is supplied (throw only if it's there, but not an object and constructor function)? Or is this considered to be too error-prone with Promise subclassing, where .then(MyCustomPromise.reject) would not work as expected (fail silently)?

# Brendan Eich (10 years ago)

a.d.bergi at web.de wrote:

Hi, I've just got bitten by

 return getError().then(Promise.reject);

where reject threw an error that its this value was not a Promise constructor.

Would it help to make the C constructor (in Promise.resolve, Promise.reject, Promise.all, Promise.race algorithms) default to Promise when no this value is supplied (throw only if it's there, but not an object and constructor function)?

No, because as you suggested:

Or is this considered to be too error-prone with Promise subclassing, where .then(MyCustomPromise.reject) would not work as expected (fail silently)?

Probably the best solution is an arrow:

 return getError().then(r =>  Promise.reject(r));
# Domenic Denicola (10 years ago)

Or just throw:

return getError().then(r => { throw r; })