How does the hash variable syntax work in typescript?

Question

...and where is it documented?

I\'ve seen examples like this around the place:

class MyThing {
  private _layers: { [id: string] : SimpleLayer } = {         


        

Answer 1

TypeScript Walkthrough: Interfaces

See the section "Describing an Indexable Object". This is called an index signature.

The syntax for defining the index is:

[Identifier: KeyType]: ValueType

KeyType can be either string or number.

You could claim that the Identifier isn't really needed since it doesn't get used anywhere, but I think it's required in order to force the class/interface designer to indicate what the hash map key should represent (an id, name, e-mail address, etc.). This also provides the possibility of having intellisense show the hash key name (as Visual Studio does for other languages), though I don't think Typescript intellisense currently provides this.