I'm getting PromiseRejectionHandledWarning: Promise rejection was handled asynchronously warning for my code, it also fails Jest tests. I've read that's Promise rejection should be handled at the same place they're defined and seem to understand the logic and the reasons. But it looks like something is wrong with my understanding as what I expect to be synchronous handling still cause warning.
My typescript code is below.
Promise, that is rejected is sendNotification(subscription, JSON.stringify(message)). From my understanding it's handled right away using .catch call, but probably I'm missing something. Can anyone, please, point me to my mistake?
private notify(tokens: string[], message: IIterableObject): Promise<any> {
const promises = [];
tokens.forEach(token => {
const subscription = JSON.parse(token);
this.logger.log('Sending notification to subscription', {subscription, message})
const result = this.WebPushClient
.sendNotification(subscription, JSON.stringify(message))
.catch(e => {
this.logger.log('Send notification failed, revoking token', {
subscription,
message,
token,
e
})
return this.revokeToken(token).catch(error => {
this.logger.error('Failed to revoke token', {
token,
error,
})
return Promise.resolve();
});
});
promises.push(result);
});
return Promise.all(promises);
}
I found the issue. In Jest you can't just mock return value with rejected promise. You need to wrap it in special workaround function:
const safeReject = p => {
p.catch(ignore=>ignore);
return p;
};
And then wrap the Promise before return it
const sendNotification = jest.fn();
sendNotification.mockReturnValue(safeReject(Promise.reject(Error('test error'))));