Typescript error - Type 'string[]' cannot be used as an index type

Question

I'm having a typescript error with my Ionic/Angular application. Below it says:

Type 'string[]' cannot be used as an index type.

Which I don't understand. The entire error is:

[17:57:57] typescript: C:/xampp/htdocs/project x/anonymous-social/src/pages/threads/threads.ts, line: 58 Type 'string[]' cannot be used as an index type.

L58: let threadId = Object(snapshot.val()[Object.keys(snapshot.val())]).threadId;

L59: let messageCount = snapshot.val().unread || 0;

My code that goes along with this is:

this.database.database.ref('/users/'+this.userData.uid+'/threads/')
.orderByChild('threadId')
.on('child_added', (snapshot) => {
      let threadId = Object(snapshot.val()[Object.keys(snapshot.val())]).threadId;
      let messageCount = snapshot.val().unread || 0;
      rec.push(Object(snapshot.val()[Object.keys(snapshot.val())]));

Any idea why this might be happening? I don't completely understand... Any help would be great!

casieber · Accepted Answer · 2018-06-07 22:05:10Z

2

In snapshot.val()[Object.keys(snapshot.val())] you are attempting to index into snapshot.val() using Object.keys(snapshot.val()).

Object.keys returns type string[].

Is there a reason you can't just use:

let threadId = snapshot.val().threadId;

I don't see the need for all that extra logic.

answered Jun 7, 2018 at 22:05

casieber

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2 Comments

Simon Over a year ago

Yes, snapshot.val() returns the first line in this image: i.gyazo.com/6eddab7527e3737cb1f72b96c040d6fe.png and the Object keys stuff returns the second line...

casieber Over a year ago

What is the expected shape of the snapshot value? It kinda sounds like you expect it only ever to have a single key-value pair and you expect Object.keys to give you the one key. But Object.keys returns an array of keys. If you are really sure it will have one, then you can change that to Object.keys(snapshot.val())[0] and TS will be okay with it, but I stand by the opinion that there is a lot of unnecessary complexity here.

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