Object is possibly 'undefined' - after filter that contains check for undefined

Question

I'm doing an operation on an array like this:

.filter(key => actions && actions[key])
.forEach(key => actions[key](event, dispatch));

But I'm getting a TypeScript error on the forEach actions: Object is possibly 'undefined'.

Yet it can't be 'undefined' as it is filtered for those entries that do have actions.

It's always possible this is a bug or short-falling of the current typescript language — theonlygusti, Feb 09 '18 at 12:41
@Andreas the TS error was specifically on `actions`, not `actions[key]` — babbaggeii, Feb 09 '18 at 13:29

score 25 · Accepted Answer · edited Feb 09 '18 at 13:09

25

I don't think the TypeScript compiler is smart enough to realize the filter method has already removed all undefined entries from the array. In this case, you can help the compiler out by telling it that actions[key] is defined by using the non-null assertion operator ! as described in this answer:

https://stackoverflow.com/a/40350534/1063392

.filter(key => actions && actions[key])
.forEach(key => actions![key](event, dispatch));

edited Feb 09 '18 at 13:09

babbaggeii

7,577
20
64
118

answered Feb 09 '18 at 12:48

Nathan Friend

12,155
10
75
125

2

This remains a problem though if the non-null assertion operator is forbidden by the lint rules in the project. – Yamo93 Sep 15 '22 at 08:49

Object is possibly 'undefined' - after filter that contains check for undefined

1 Answers1